Annoying problem with apache22 / php5 - how to investigate?

[Per olof Ljungmark]

Hi,

We run 7-STABLE and apache22 with php5 serving pages from a webmail app 
(Horde).

Randomly (as it seems at least), there is a 500 (Internal server error) 
and a blank page is presented to the user like

[04/Jul/2009:15:19:37 +0200] "GET 
/services/portal/sidebar.php?httpclient=1 HTTP/1.1" 500 -

There are no other messages in the logs, not even with LogLevel debug. 
The problem has survived several both OS and port upgrades and I really 
need to track this down now.

Question: What OS tools would be the best to further analyze this? 
Someone with more exparience running this combo perhaps would know?

Thanks a lot!

--
per