unable to send mail from IMP 2.2.7

[Ben Elliston]

I am unable to send mail from IMP.  When I hit the "Send" button, the
composition window is refreshed and I get a blank new message form to
complete--the message being edited is lost and is never sent.  I am
running:

  horde-shm-1.2.7-1rh7
  horde-1.2.7-1rh7
  imp-2.2.7-1rh7
  php-4.0.6-9.7.1
  php-imap-4.0.6-9.7.1
  php-pgsql-4.0.6-9.7.1

I use Postfix as my MTA and have confirmed that /usr/sbin/sendmail 
user@domain delivers mail correctly.

Is this a known problem?  What can I do to track it down?  Thanks,

B.