Message expiry problem

I use Expiry on most of my (IMAP) folders. On less important ones, expiry permanently deletes messages. That works fine. More important folders, I set the expiry to move them to server/Trash, where they stay a little longer before being permanently deleted. At least, that has always worked before. Now I'm seeing, on the F10 netbook "Moving old messages from folder KDE-fedora to folder Trash failed". My 4.1.3 laptop can still expire them, so although I've not been aware of when this started it's probably quite recent. Anne