Thx to everyone who tried to help
I've found the culprit.
It's Windows-Defender
In the Ransomware-Protection, the 3 OneDrive-Folders i was testing with (Documents, Pictures, Desktop) are protected.
Could even see it in the Access-History.
Though i have to say, the Error-Message sent us on a merry chase....
*sigh*....now i have to negotiate with IT-Department....
this is going to be fun....