I’ve had this happen a couple of times in the past few weeks. Somehow, something in the project is getting corrupted. I’ve recovered from this by calling up the last, good, autosaved version and going from there. It will then allow you to save that version. I’ve been working with .PSD files each time the problem has occured.
In a post below, within the last week or two there was a suggestion for increasing the memory allocation to the still cache as a solution to the bug. I tried it, and haven’t had the problem since. Just lucky? Maybe…but it might be worth a try, anyhow.
Tom