AE 2021 suddenly crashes every time I try to save the project - not the obvious causes...

I was running AE2021 version 18.4.1,working on a modestly sized project. About 12 Comps, most with about 4-10 layers. Using Mocha Pro extensively. I have been working on the project all day, saving regularly - no problems. I hit save while a RAM preview is running (not usually an issue). The RAM preview keeps chugging away through the frames while the saving progress notice comes up. Then the project crashes. From then, I am not able to save the project.

I have tried:

- clearing all memory and disk cache - still crashes

- saving to a new location with a new name - crashes

- going back to an earlier autosave version of the project, and saving that - crashes

- deleting the comp I was working on and saving a new version from there - crashes

- updating to version 18.4.2 (don't want to make the 2022 upgrade in the middle of the project, because I will have to reinstall plugins, which is time-wasting pain - but then so is this) - still crashes

- uninstalling, and then re-installing - still the same

Other projects still open and save fine. Somehow this one just suddenly lost it's s*** for no discernible reason.

Ryzen 3900X

1080ti

64GB RAM

m.2 drives - rushes on seperate drive to project files

Any thoughts? The frequency of stuff like this with Adobe software seems on the up and up!