Network Sharing issues in windows server 2012

I am an IT Administrator in a Government Office and the office has one Dell PowerEdge T310 Workstation with Windows Server 2012 R2 installed on it. It's the Domain Controller for the office with File and Storage service installed and configured with folder sharing purpose for different departments in the office to share files and store it on server.

This was how it was when I was hired in the office and now I am the only one who is controlling and configuring it. For a long time it has been working smoothly and I was also adding new computers, users and printers.

One day I tried to use a shared printer from one computer to another computer and it started showing me different errors. I checked everything and it was normal, but I kept getting that error whenever I tried to access shared files and folders on the server or another computer and also the shared printers.

It works while I try it too many times and suddenly again it shows me that error. When I try it repeatedly, it opens and then again after a few seconds or minute it shows me different errors.

This is how the network looks:

This is the error that it shows me when I tried to access a computer:

Windows cannot access \\computername

Check the spelling of the name. Otherwise, there might be a problem with your network. To try to identify and resolve network problems, click Diagnose.

Error code: 0x80070035
The network path was not found.

After trying to open it again and again, then it opens and shows me the shared folders on the server.

Then after a few seconds it shows me an empty folder and another error again:

It's also the same when I opened other computers other than the server:

After a few tries it opens with the shared printer and folders in that computer, but I can't print over the network and can't access the shared files from another computer.

This is what my problem looks like. Does anyone knows anything about it?