IBM Htttp Server 1.3.26 500 errors

We are running IBM Http Server 1.3.26 and IBM Appl. Server 5.0.1 on a
Linux Advanced Server 2.1. Also, Web Server is a SSL enabled. Every
fifteen minutes we run a script that submits one URL to the system,
just to see if it is any problem. We are getting, even if it is no
users on the system, 500 errors, but nothing has been recorded in
Appl. Server log files, only web server log file has an entry with a
500 error. For now, we can not find any pattern when this happening.
Few seconds after we got 500 error, a new users sends a request to the
web server and get 200 response.

This is a production problem. Thanks for the help.