Hi, I had this same issue and I found that it was related to saving a very large selection set. About 40 slots with an average of 3 images per slot, and translate / scale / rotate / sheer and constraints and etc. Basically saved a selection for all things that are possible to key.

My solution for programming purposes is to key every single aspect of the dope sheet on the first and last frames of every animation. Therefore, I made a selection set so I could easily select slots, attachments, bones, etc whether they are on or off(thats the most important part)

Anyway, when I went to save it wouldn't let me, it would give me "An error occurred while saving the project"

I was able to fix this problem after I saved over the selection set with a much smaller amount of things selected (like 3 bones only) After I did that it allowed me to save again.

I'm guessing it was just too much data to save.

I don't know if this is related to your issue but this was my issue and how I solved it.