Generally this indicates either a template issue or a mysql statement gone bad.
Is this on a machine local to you, or a remote server? Seeing as your using apache, the following should work if your not on win 95:
php_value php_error_log "syslog"
All PHP errors will then be pushed to your system log for debugging.
If your on a remote machine where you do not have access to the syslog, please let me know, and I'll get you a new .htaccess. That one would be more complicate. Trick is finding where the error occurs exactly.