When I hit `g', Edwin hung while trying to draw the mode line with the error that %GET-MESSAGE was returning #F in COUNT-UNSEEN-MESSAGES.  Somehow, the index exceeded the length of the folder, suggesting that FIRST-UNSEEN-MESSAGE-INDEX returned a number exceeding the folder's length, which suggests in turn that the IMAP code processed an UNSEEN reply before the folder was fully updated, and that Edwin tried to redraw the mode line in that window.  No IMAP trace or transcript available, unfortunately.

Several stopgap measures should work around this: write (>= i n) rather than (= i n) in COUNT-UNSEEN-MESSAGES, change the handler for UNSEEN replies to take the minimum of the folder's length and the new unseen value, &c.