"Could not save project document error" persisting even on local drive

Hi - have been using Scrivener 3 without issue for the past few weeks, but today I got the following error:

[attachment=0]Screenshot 2021-05-12 202752.png[/attachment]

I’ve seen others on here post that this was fixed for them by saving somewhere other than cloud storage, but after saving a copy of the project to the desktop (as in the screenshot) the problem still persists. Can anyone tell me what’s going on here? I’ve tried launching Scrivener in administrator mode and it makes no difference.