Suddenly can't save AI files after manually uploading backup to OneDrive
I don't sync to OneDrive automatically, I manually upload my files via browser (copying them for backup). I frequently upload the current file of an Illustrator project while I'm working on it. Then I continue in Illustrator as normal, working and saving more, uploading again periodically, without any issue. This has worked fine for several years. Suddenly the last few days, after I manually upload the current file to OneDrive, I can no longer save any future changes in Illustrator. I get the warning "The file may be read-only, or another user may have it open. Please save the document with a different name or in a different folder". I haven't changed anything.
This is very inconvenient. I'm hoping someone knows if this is a OneDrive or Windows issue that has just started (like there's still something tagging that file as being in use even after the upload is complete), or if it is an Adobe account or Illustrator issue. And of course I'd like to know how to fix it!
System & what I tried already:
Windows 10 PC (private, with full administrator permission account).
I tried closing the OneDrive browser tab (I access it via browser, I sign into that separately from my Windows computer account). I tried closing the Windows Library I had open where the file is located. I tried disconnecting my VPN. I tried closing the file in Illustrator then opening it again. I tried closing Illustrator itself and starting it again and opening the file again, then tried saving changes. None of those made any difference. I restarted my computer, signed out of OneDrive and logged back in, opened Illustrator, opened a file, made a change, finally it saved fine, but then after uploading to OneDrive, again I can no longer save in Illustrator. The files in question are located on my PC's hard drive. Again, I haven't changed anything. The only thing I can do now is save it with a different file name after every time I've uploaded to OneDrive. To save confusion, I delete the original file and rename the current file what it should be named. Then work on that file more until I upload a backup to OneDrive again, and repeat the process. Ugh!
