[imp] IMP 3.1 login issues

[Issac Goldstand]

Hi,
  I am having a strange problem with IMP.  I just upgraded from an old Horde
1/Imp 1 setup to Horde 2.1/IMP 3.1/Turba 1.1
I am using IMP as the login mechanism, which I think I set up OK as I am
prompted for mail server information on the main horde login page.  I am
authenticating OK to the local IMAP server as I can see in the
/var/log/messages file.  Horde's sessions seem to be working fine from the
test.php page.  However, after clicking "login", I'm immediately returned to
the login page.  Each element of the authentication process *seems* to be
working.  Any ideas of what the issue might be?

Thanks,
  Issac