Annoying problem with apache22 / php5 - how to investigate?

[Bruce Ferrell]

Per olof Ljungmark wrote:
> 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

I'd look in the Apache error_log