ESP8266 repeatedly throws Exception(0)

I am making an IoT weighing scale with a load cell connected to an Arduino. The Arduino then sends data to an ESP8266 connected by serial which would subsequently update my database on Google's Firebase. However, my ESP8266 throws an exception when it starts. Half of the time it works perfectly fine. The other half throws the following error when I connect my serial cable. This error runs repeatedly so I think it might be an error inside void loop() but I can't seem to figure out what exactly is the problem. Also, my cables are not the best quality but I am not sure if that affects anything

Exception (0):
epc1=0x4025803e epc2=0x00000000 epc3=0x00000000 excvaddr=0x0000000a depc=0x00000000

>>>stack>>>

ctx: sys
sp: 3fffe0c0 end: 3fffffb0 offset: 01a0
3fffe260:  00000000 00000000 00000000 00000000  
3fffe270:  00000000 00000000 00000000 40104b72  
3fffe280:  40104b54 3fffc100 0000001a 00000000  
3fffe290:  00000000 4025803e 00000000 00000000  
3fffe2a0:  400005e1 00000000 00000000 00000000  
3fffe2b0:  4025803e 00000033 00000010 00000000  
3fffe2c0:  4025803e 40266fd0 00000000 00000001  
3fffe2d0:  fbf8ffff 04000002 3feffe00 00000100  
3fffe2e0:  0000001a 00000018 04000102 40104b54  
3fffe2f0:  3fffc100 0000001a 00000000 00000000  
3fffe300:  00000000 00000000 00000000 00000000  
3fffe310:  00000000 00000000 00000000 00000000  
3fffe320:  00000000 00000000 00000000 00000000  
3fffe330:  00000000 00000000 00000000 00000000  
3fffe340:  00000000 00000000 00000000 00000000  
3fffe350:  00000000 00000000 00000000 00000000  
3fffe360:  00000000 00000000 00000000 00000000  
3fffe370:  00000000 00000000 00000000 00000000  
3fffe380:  00000000 00000000 00000000 00000000  
3fffe390:  00000000 00000000 00000000 00000000  
3fffe3a0:  00000000 00000000 00000000 00000000  
3fffe3b0:  00000000 00000000 00000000 00000000  
3fffe3c0:  00000000 00000000 00000000 40104b72  
3fffe3d0:  40104b54 3fffc100 0000001a 00000000  
3fffe3e0:  00000000 4025803e 00000000 00000000  
3fffe3f0:  400005e1 00000000 00000000 00000000  
3fffe400:  4025803e 00000033 00000010 00000000  
3fffe410:  4025803e 40266fd0 00000000 00000001  
3fffe420:  fbf8ffff 04000002 3feffe00 00000100  
3fffe430:  0000001a 00000018 04000102 40104b54  
3fffe440:  3fffc100 0000001a 00000000 00000000  
3fffe450:  00000000 00000000 00000000 00000000  
3fffe460:  00000000 00000000 00000000 00000000  
3fffe470:  00000000 00000000 00000000 00000000  
3fffe480:  00000000 00000000 00000000 00000000  
3fffe490:  00000000 00000000 00000000 00000000  
3fffe4a0:  00000000 00000000 00000000 00000000  
3fffe4b0:  00000000 00000000 00000000 00000000  
3fffe4c0:  00000000 00000000 00000000 00000000  
3fffe4d0:  00000000 00000000 00000000 00000000  
3fffe4e0:  00000000 00000000 00000000 00000000  
3fffe4f0:  00000000 00000000 00000000 00000000  
3fffe500:  00000000 00000000 00000000 00000000  
3fffe510:  00000000 00000000 00000000 40104b72  
3fffe520:  40104b54 3fffc100 0000001a 00000000  
3fffe530:  00000000 4025803e 00000000 00000000  
3fffe540:  400005e1 00000000 00000000 00000000  
3fffe550:  4025803e 00000033 00000010 00000000  
3fffe560:  4025803e 40266fd0 00000000 00000001  
3fffe570:  fbf8ffff 04000002 3feffe00 00000100  
3fffe580:  0000001a 00000018 04000102 40104b54  
3fffe590:  3fffc100 0000001a 00000000 00000000  
3fffe5a0:  00000000 00000000 00000000 00000000  
3fffe5b0:  00000000 00000000 00000000 00000000  
3fffe5c0:  00000000 00000000 00000000 00000000  
3fffe5d0:  00000000 00000000 00000000 00000000  
3fffe5e0:  00000000 00000000 00000000 00000000  
3fffe5f0:  00000000 00000000 00000000 00000000  
3fffe600:  00000000 00000000 00000000 00000000  
3fffe610:  00000000 00000000 00000000 00000000  
3fffe620:  00000000 00000000 00000000 00000000  
3fffe630:  00000000 00000000 00000000 00000000  
3fffe640:  00000000 00000000 00000000 00000000  
3fffe650:  00000000 00000000 00000000 00000000  
3fffe660:  00000000 00000000 00000000 40104b72  
3fffe670:  40104b54 3fffc100 0000001a 00000000  
3fffe680:  00000000 4025803e 00000000 00000000  
3fffe690:  400005e1 00000000 00000000 00000000  
3fffe6a0:  4025803e 00000033 00000010 00000000  
3fffe6b0:  4025803e 40266fd0 00000000 00000001  
3fffe6c0:  fbf8ffff 04000002 3feffe00 00000100  
3fffe6d0:  0000001a 00000018 04000102 40104b54  
3fffe6e0:  3fffc100 0000001a 00000000 00000000  
3fffe6f0:  00000000 00000000 00000000 00000000  
3fffe700:  00000000 00000000 00000000 00000000  
3fffe710:  00000000 00000000 00000000 00000000  
3fffe720:  0af8a400 00000001 647a4f3a 00000000  
3fffe730:  0af8a400 00000001 647a4f3a 00000000  
3fffe740:  00000000 00000000 00000000 00000000  
3fffe750:  00000000 00000000 00000000 00000000  
3fffe760:  00000000 00000000 00000000 00000000  
3fffe770:  00000000 15f14800 000015f1 00004800  
3fffe780:  0000c8f4 000215f1 00000000 00000000  
3fffe790:  c8f49e74 00009e74 00000000 00000000  
3fffe7a0:  00000000 00000000 00000000 00000000  
3fffe7b0:  00000000 00000000 00000000 40104b72  
3fffe7c0:  40104b54 3fffc100 0000001a 00000000  
3fffe7d0:  00000000 4025803e 647a4f3a 00000000  
3fffe7e0:  400005e1 00000002 000000ad 00000000  
3fffe7f0:  4025803e 00000033 00000010 004e11a5  
3fffe800:  4025803e 40266fd0 00000000 00000001  
3fffe810:  fbf8ffff 04000002 3feffe00 00000100  
3fffe820:  0000001a 00000018 04000102 40104b54  
3fffe830:  3fffc100 0000001a 00000000 00000000  
3fffe840:  ffffffff 00000000 00000001 00000000  
3fffe850:  00005201 00000000 4026492a ffffffff  
3fffe860:  00000000 fffffffd 00000000 00000000  
3fffe870:  00000000 00000000 00000001 40006545  
3fffe880:  00000000 00000000 00000000 40006545  
3fffe890:  00000001 00000001 00000000 40006589  
3fffe8a0:  00000000 3fffe922 00000000 40264b1e  
3fffe8b0:  40264b32 00000001 00000000 00000000  
3fffe8c0:  3fffe8d3 00000000 00000000 40006545  
3fffe8d0:  000005e0 3fffe918 3ffef288 40226a30  
3fffe8e0:  00000608 00000000 00000000 00000000  
3fffe8f0:  00000000 00000020 402532c3 00000001  
3fffe900:  ffffffff 00000000 3ffe8e91 40104b72  
3fffe910:  40104b54 3fffc100 0000001a 00000000  
3fffe920:  00000000 4025803e 00000020 401001a4  
3fffe930:  400005e1 00000000 00000000 401015bc  
3fffe940:  4025803e 00000033 00000010 3ffed11c  
3fffe950:  4025803e 40266fd0 00000000 00000001  
3fffe960:  fbf8ffff 04000002 3feffe00 00000100  
3fffe970:  0000001a 00000018 04000102 40104b54  
3fffe980:  3fffc100 0000001a 00000000 00000000  
3fffe990:  00000000 00000002 3ffee8a0 402660a3  
3fffe9a0:  c0266089 7fffffff 00000002 40211114  
3fffe9b0:  3ffe8765 00000002 3ffee898 4026608a  
3fffe9c0:  00000000 7fffffff 3fff17ec 3ffed29c  
3fffe9d0:  00000000 3ffeea48 3ffe8765 40222cde  
3fffe9e0:  3fffea00 3ffeea1c 3ffe8765 3ffeea78  
3fffe9f0:  3ffe8765 3ffeea9c 3ffe8765 40222cde  
3fffea00:  3fffea20 3ffeebb4 3ffeeb24 3ffed29c  
3fffea10:  3ffe8765 00000000 3ffee678 402063e8  
3fffea20:  40247cf1 00000000 3ffeeaa8 402037d8  
3fffea30:  ffffffff 00000000 3ffeeaa8 402109af  
3fffea40:  00000000 00000000 00000001 401001a4  
3fffea50:  3ffedffc 3ffee028 3ffe8abc 40104b72  
3fffea60:  40104b54 3fffc100 0000001a 00000000  
3fffea70:  00000000 4025803e 00000001 402587be  
3fffea80:  400005e1 00001000 40104e0d 000003fd  
3fffea90:  4025803e 00000033 00000010 00000000  
3fffeaa0:  4025803e 40266fd0 00000000 00000001  
3fffeab0:  fbf8ffff 04000002 3feffe00 00000100  
3fffeac0: c_⸮⸮rS⸮

This is my code on the ESP8266. I apologize for any bad programming habits.

#include <ESP8266WiFi.h>
#include <FirebaseESP8266.h>

//Hidden for security
#define WIFINAME   
#define WIFIPASSWORD
#define BAUTRATE 9600
#define FIREBASE_HOST  
#define FIREBASE_COMMAND_ADDRESS
#define FIREBASE_AUTH 

WiFiClientSecure client;
FirebaseData firebaseData;
unsigned long previousTime;
String firebaseWriteAddress;
String firebaseReadAddress;
String weight; // Stored in a string to accomodate the "loading" text
bool maxWRead;
bool minWRead;
bool displayPrefRead;
bool instructedToConnectWifi;

void readWeights(){
  // function to read the min and max weights, then send to Arduino

  if (!maxWRead){
    String maxWAddress = firebaseReadAddress + "/Product/MaxW";
    if (Firebase.get(firebaseData,maxWAddress)){
      if (firebaseData.dataType() == "string"){
        String maxWMessage = "max:"+ firebaseData.stringData();
        Serial.println(maxWMessage);
        maxWRead = true;
      }
    }
  }

  if (!minWRead){
    String minWAddress = firebaseReadAddress + "/Product/MinW";
    if (Firebase.get(firebaseData,minWAddress)){
      if (firebaseData.dataType() == "string"){
        String minWMessage = "min:"+ firebaseData.stringData();
        Serial.println(minWMessage);
        minWRead = true;
      }
    }
  }
}

void readDisplayPref(){
  String displayAddress = firebaseReadAddress + "/Display";
  if (Firebase.get(firebaseData,displayAddress)){
    if (firebaseData.dataType() == "string"){
      String displayMessage = "dsp:"+ firebaseData.stringData();
      Serial.println(displayMessage);
      displayPrefRead = true;
    }
  }
}

void sendFireBase(){
  // function to send data to Firebase Real-time Database
  Firebase.setString(firebaseData, firebaseWriteAddress,weight);
}

void readFireBase(){
  Firebase.begin(FIREBASE_HOST,FIREBASE_AUTH);
  if(Firebase.get(firebaseData, FIREBASE_COMMAND_ADDRESS)){
    if (firebaseData.dataType() == "string"){
      String data = firebaseData.stringData();
      String command = data.substring(0,4);
      if (command == "read"){ 
        firebaseReadAddress = "/UserData/" + data.substring(4);
        readWeights();
        firebaseWriteAddress =  "/UserData/" + data.substring(4)
        + "/Weight";
      }
    }
  }
}

void connectWifi(){

  WiFi.begin(WIFINAME, WIFIPASSWORD);

  byte wifiCounter = 0;
  // Allow some buffer time to connect to WiFi
  while (WiFi.status() != WL_CONNECTED) {
    delay(100);
    wifiCounter ++;

    if (wifiCounter > 50){
      // tries for 5 sec.
      break;
    }
  }

  if (WiFi.status() == WL_CONNECTED){
    Serial.print("WiFi Connected!\n");
    instructedToConnectWifi = true;
  }

  else {
    Serial.print("Can't Connect. Trying Again\n");
    connectWifi();
  }
}

void readArd(){
// function to read Serial inputs from arduino
  String message = Serial.readStringUntil('\n');
    if (message == "ConnectWifi"){
      connectWifi();
    }
    else{
      weight = message;
      // stores weight without the percentage sign. Only stores if its within 0-100
      sendFireBase();
    }
}

void setup() {
  Serial.begin(BAUTRATE);
  previousTime = millis();
  maxWRead = false;
  minWRead = false;
  displayPrefRead = false;
  instructedToConnectWifi = false;
}
 
void loop() {

  if (Serial.available()){
    readArd();
  }
  
  if (instructedToConnectWifi){
      // if wifi disconnects halfway, auto connect back
      if (WiFi.status() != WL_CONNECTED){
      connectWifi();
    }
  }
  
   unsigned long currentTime = millis();
  if (currentTime - previousTime >2000){
    // cycles every 2 sec to prevent spam
    // TODO: fix overflow problem (49 days) if it becomes product
    
    if (instructedToConnectWifi){
      if (!minWRead || !maxWRead){
        // stop once max and min has been read
        readFireBase();
        previousTime = currentTime;
      }
      else {
        readDisplayPref();
        previousTime = currentTime;
      }
    }
  }
}

These exeption analysed gives me the following result:

Exception 0: Illegal instruction

looking deeper into the error reveals:

std::basic_string , std::allocator  >::basic_string(char const*, std::allocator  const&) 

This error pattern is typical for the String class.
Replace all Strings with fixed char arrays (not dynamicly created ones(!))
Reason: the String class tends to fracture the heap which causes the ESPs to crash (8266 as well ESP32).
Why those? Because we use them (opposite to "normal" Arduinos to 99% in WiFi scenarios (IoT). So there is a lot happening in memory.
What to do? Get rid of String class (and delay() if used)

String firebaseWriteAddress;
String firebaseReadAddress;
String weight; // Stored in a string to accomodate the "loading" text

These above in combination with functions like (dynamicly defined String vars)

String data = firebaseData.stringData();
  String command = data.substring(0,4);
  if (command == "read"){ 
    firebaseReadAddress = "/UserData/" + data.substring(4);
    readWeights();
    firebaseWriteAddress =  "/UserData/" + data.substring(4)
    + "/Weight";

will ensure that the ESP crashes very quickly. Since there is no garbagge collection (and would have not enough memory anyway) you write at some point into occupied memory - see first line of my error analysis - crash.
Why work fixed chars? - they are compiled into flash and you operate most of the time with pointers. So all "fixed" text messages go into flash (example)

char firebaseReadAddress[64] = {'\0'}; //Takes 63 chars

used like

 strcpy(firebaseReadAddress, "/UserData/");
 strcat(firebaseReadAddress, dataContent);

to check wether a char array starts with "read" you use this method:

 if (strncmp( command, "read", 4) == 0) {  /** Compare the first 4 characters - returns 0 if they are equal */

The handling is as easy as String class, but it lets your app run forever (at least as long there is a power source)
Check also used libraries for heavy String use (especially if they are not dedicated for ESPs). If they do - ditch them or rewrite them ;-)
The firebase lib is a positive example no use of dynamic Strings and heavy use of char arrays.
Sorry for the bad news
Rewrite your program if you ever want a stable app, I can assist you if there are special questions - just leave a comment here.