Any news on this?  I seem to get this error every few hours on my i7 3930k (Win 7 x64) but not on any of my other machines with various Intel processors.  A simple restart and it goes away, but if I'm not at my computer then I lose mining time until I can get back.  If I had to make a wild guess, it's asserting that the thread pool has nothing but idle threads, but I've not waded through the source to find what the problem is.