Python crashes when using call (ntdll.dll)

I have a script that combines around 1000 files by using an external program. It does so by copying the first file to combined.sym and then combining the this file with all other files, one by one. Code is as follows:

# gets all .sym files
symfiles = []
onlyfiles = [f for f in listdir(myPath) if isfile(join(myPath, f))]
for f in onlyfiles:
    if '.sym' == splitext(f)[1]:
        symfiles.append(f)

# copies one initial combined file (point to start from)
combinedFileName = "combined.sym"
copyfile(myPath + symfiles[0], myPath + combinedFileName)
for sym in symfiles:
    print("processing " + str(sym))
    cmdString = combinedFileName + " "
    cmdString += sym
    cmdString += " -a " + combinedFileName
    finalCmdString = r'cmd.exe /C fuseFiles.bat "' + myPath + '" "' + cmdString + '"'
    call(finalCmdString)
print("done")

The batchfile I'm calling looks like this, but that is not the issue:

@set PATH=\path\to\executable\;%PATH%
@cd %1
@FusingProgram "%2"

Now, when I run this script, it runs for like 15 Minutes before it just stops and returns to the CMD.

processing onefile.sym
processing anotherfile.sym
processing evenmorefiles.sym
processing yougetit.sym

C:\Users\test\>

As you can see, it misses the "done" print, and it didn't process all files. The combined.sym is created and expanded by the other .sym files

When checking the Windows events, I can see, that it logs python as crashed and lists ntdll.dll as faulty module(sorry for german log):

Name der fehlerhaften Anwendung: python.exe, Version: 0.0.0.0, Zeitstempel: 0x53787196
Name des fehlerhaften Moduls: ntdll.dll, Version: ****, Zeitstempel: 0x5b6db230
Ausnahmecode: 0xc0000005
Fehleroffset: 0x0002e2eb
ID des fehlerhaften Prozesses: 0x6224
Startzeit der fehlerhaften Anwendung: 0x01d454e0ff20377c
Pfad der fehlerhaften Anwendung: C:\pat\to\python\python.exe
Pfad des fehlerhaften Moduls: C:\WINDOWS\SysWOW64\ntdll.dll
Berichtskennung: 8b4254f0-c0d5-11e8-9a6f-901b0e5a6206

I've done some research and it seems like I'd have to resolve this by updating the ntdll.dll file, but my system is all up to date, so there is nothing I can do about that (except for downloading another version from some untrustworthy site, which I'm not gonna do)

All this being said, have you ever encountered a similar issue or is this just a stupid mistake I made?