There are at least two more threads with similar problems:
Cannot save files, not even as copy with new name (I created this one 10 days ago)
This file cannot be saved or created (similar problem on MacOS 4 days ago)
@Galvinizer 's post is quiet harsh, but he’s right in saying that this is a serious issue. Saving your files is an essential feature for an editor and I can’t hope to convince my CEO to switch to OnlyOffice from MS if she can’t save her files. (In my case the saving doesn’t work and modifications are lost.)