Started using gnubiff earlier this week, and I'm sure it worked originally, but can't be certain.
My problem is, I setup gnubiff to monitor my local mail, /var/mail/todd, then close the Preferences window, then when I right click on the Gnome panel icon, and select Preferences, gnubiff hangs up. The preferences window doesn't open, and the applet becomes unresponsive, and I have to kill it.
First, I erased my gnubiff configuration (.gnubiffrc), thinking wonky in the config file, set it up again, and still hangs. I then thought maybe one of the e-mails in my inbox was causing it grief, so got rid of all my mail in my inbox (i.e. /var/mail/todd was 0 bytes after that), and problem still occurs.
I'm also monitoring two IMAP mailboxes and I have no problems with them causing gnubiff to hang.
sorry for the delayed reply. It seems sourceforge didn't notify me when you posted your message.
The problem you encountered is probably the same as the one in bug report #1221040. This bug is fixed in the CVS version of gnubiff and upcoming gnubiff 2.1.5 (which will probably released next weekend).
As a workaround you can create a valid .gnubiffrc file using the gtk version of gnubiff.