[imp] Horde login supposedly fails, but clicking on INBOX works fine

[Sun Advocate Webmaster]

I'm having an odd problem. On some occasions, upon logging in to horde, 
I get "login failed". What is unusual is that I see the left navigation 
pane. I can click on mail (or the INBOX inside mail), and my inbox 
appears correctly.

See here:
http://www.sunad.com/public/horde-login.png

I'm not using any sessions handler (as set up in my horde config), but I 
have had PHP sessions enabled in the past. That setting doesn't seem to 
affect this issue any. It is supposed to show my inbox upon login.

I checked the logs, and it says:
Jun 07 18:48:05 HORDE [notice] [imp] 10.0.0.90 failed [on line 31 of 
"/path/to/horde/imp/login.php"]

On line 31 of login.php, it says:
Horde::logMessage($_SERVER['REMOTE_ADDR'] . ' ' . $logout_reason, 
__FILE__, __LINE__, PEAR_LOG_NOTICE);

I'm not sure where to go from here. This is a major snag to our overall 
adoption of Horde, and I'm stuck!

Any ideas?