[imp] Redirect loop on IMP auth

[Jaz]

IMP, at authentication, is cycling the following into my Apache2 log in 
an endless loop. This is after upgrading Apache from 1.3 to 2.0.54 and 
PHP from 4.3 to 5.0.4 (and PEAR). I have hordeauth set to true in 
imp/servers.php -- set to false the result is a simple IMP login failure.)

172.16.30.101 - - [28/Jun/2005:19:43:04 -0400] "GET 
/horde/imp/redirect.php?actionID=login&autologin=1 HTTP/1.1" 200 26
172.16.30.101 - - [28/Jun/2005:19:43:05 -0400] "GET /horde/imp/login.php 
HTTP/1.1" 302 26
172.16.30.101 - - [28/Jun/2005:19:43:06 -0400] "GET 
/horde/imp/redirect.php?actionID=login&autologin=1 HTTP/1.1" 200 26
172.16.30.101 - - [28/Jun/2005:19:43:06 -0400] "GET /horde/imp/login.php 
HTTP/1.1" 302 26

The browser keeps refreshing (redirecting) every 2-3 seconds until I hit 
stop or Escape. Each redirect is logged as above.

This horde/imp/etc environment had been working fine for months, the 
only change is Apache2 + PHP5 + PEAR.

I realize this is likely an Apache or PHP config problem, but I'm hoping 
horde/IMP pros might recognize the behavior. Several other PHP-driven 
apps work fine (as do other horde modules) on this new Apache2/PHP5 setup.

Worth noting is that although Mcal is no longer a separate extension and 
is supposedly bundled into PHP5, horde's test.php script reports no 
support for Mcal -- the same is true for domxml (again, supposedly). 
Please see http://harbell.homeip.net/horde/test.php.

I upgraded because Chuck pointed me to a bug in PHP4 on win32 that 
causes regeneration to strip the version line from config's. This 
problem is gone, tho I hope the Mcal, domxml, etc. differences don't 
prevent me from using this environment.

Any advice on the cause or how to attack/debug?
-Jaz