Error message ORA-03113 and client certificate

[roymoor 0]

Hello there,

I'm running Apache 2.0.55 with PHP 5.0.4. After a couple of hours, my PHP script shows an Oracle error message when trying to establish a connection:
Warning: ocilogon() [function.ocilogon]: OCISessionBegin: ORA-03113: end-of-file on communication channel in /app/test/htdocs/sso/include/oracle.inc on line 47

Warning: Cannot modify header information - headers already sent by (output started at /app/test/htdocs/sso/include/oracle.inc:47) in /app/test/htdocs/sso/include/upcsession.inc on line 11

After stopping and starting apache, the error disappears for a while.
The error only occurs when I'm connecting to Apache with my client certificate. Everything works fine when opening a location that does not require client certificates.

I've got Oracle 8.1.7. installed on my server.

Hope someone can help me out.