Doubtless, someone else will have a better answer. Have you been using the quick save? If so, that could be why. It corrupts save files. If not...then I don't know.
I got this bug once and was able to fix it by using right click to talk to my wizards familiar. I don't know why that fixed it, but it did.
It almost sounds like a new version of the "secret door" bug, not really sure. Check the bug thread lists in this forum.
http://www.co8.org/forum/showthread.php?t=6119 Exact steps I took to fix the issue originally described. I hope this helps.