Hi,
I’m not sure that the cause of the problem is this one, but is it possible that the user has to explicitly authorise your app to make changes in specific folders? I have seen it happening. It’s worth checking if Windows Defender’s “Controlled Folder Access” is blocking app permissions to write files.
It’s of course also possible that this is yet another funny behaviour of OneDrive, whose existence has brought pain to thousands of developers…
Andrea.