[imp] blank page

[Stacy Dunkle]

Horde 3.0.4
Imp 4.0.3
Turba 2.0.2
PHP 5.0.4-10
Apache 2.0.54
Fedora Core 4
authentication through Imp

I'm testing out a new installation of Horde/Imp, after previously  
using the Horde2/Imp3 series for a while on another server.

While having various people log in to the test install, I'm having an  
issue where some people can login and get their inbox just fine,  
while others just get a blank page, stuck on the redirect.php url.

I've tested it on various browsers and machines, with the same mixed  
results. I've even cleared the caches, deleted cookies, etc., to no  
avail.

Has anyone else ran into this problem? Is there a way to fix this?  
Should I not be using PHP 5?

-Stace