Actually, the full error message I have been getting was “Summary and folder mismatch, even after a sync” and “Unable to store Inbox.” I found this older post, which I include because of the valuable suggestion that one backup one’s mail folders before trying the fix, and also because I believe some people may need to delete the additional files he indicates.
I did not need to delete the files indicated, however, I had to delete folders.db, also in folder “local.” That fixed the problem.
Always, always back things up before deleting files in this fashion!
