Thank you guys for the informations.
We found what the issue is and yes Gerry, it is what you've mentioned, that there is a missing protection against a Cycle start while applying and saving.
I will add this protection now.
And we could finally reproduce it with my collegue removed the Windows Defender exclusion for the UCCNC folder on his Win10 machine and when Windows defender is enabled on the UCCNC folder then it slows down the saving and that way he could reproduce the issue.
So, by the way, if you guys running Windows 10 then add an exclusion for the UCCNC folder in Windows defender, because it can really slow down the saving process.