'Unable to open project file' error messages

I have spent many, many hours on my current project with building and previewing, getting no error messages whatsoever. Now I come to do the last few edits ready for it to be published to students and I cannot progress for 'Unable to open project file' or other error messages which seem often to be random in nature: I get access to a project for a while then when I try to open it minutes later, I can't. I could understand it if I was moving files or changing their names but I have kept the same file names and folder structure from the start.

Rebooting my computer, shortening file names on a test piece (to well under the limit), files being on my hard drive, deleting the .ppta file.... none of this has any effect. This has happened before but has usually come right all by itself in a couple of days but now I am completely stuck, unable to edit/preview this work and running out of time.

I can edit and preview Engage interactions outside Presenter so this appears to be a problem with Presenter in particular. And though this means I can create/edit Engage files, I cannot reliably put them together using Presenter (that is, I can create something in Presenter but not be confident I can edit/preview it later).