I have the same problem - started earlier this afternoon. In addition to not letting me empty trash, it won't let me move any emails to trash either. My mailbox is nowhere close to being "full", being only at 43%. And it was actually was at 44% earlier today when I happened to easily delete about 1% to bring it down to the current 43% before this problem set in this afternoon. Closing Comcast email & rebooting had no effect. Same problem when trying to use Comcast email on either of my computers as well as on my iPhone.
Try clearing your browser cache, then close your browser and then start a new session.