So in short... there is this App called ENVDIR which copies texture files from it's folders to another App's folder.
ENVDIR kept failing in succesfully move those files and after a many hours of struggle (also involving the ENVDIR Developer over TeamViewer) we concluded that BitDefender somehow interfeeres with ENVDIR function.
I started to investigate an although I have set up some AV exclusion for the App, the Ransomware protection kicked in randomly without any notice.
Only after I added the special exclusion in the Ransomware module for ENVDIR, ENVDIR worked as intended.
To conclude: Why would BD interfeere with Apps without notice?