We were ended-up in this message after running EasyApache on one of our cPanel servers with mod_suphp. If we set the PHP handler to DSO, PHP applications were working fine; but when we try to switch the handler to SuPHP it started showing the above error message with 500 error code.
The actual cause for this error is given in the messsage in fact, which is “Calling user is not webserver user”. As per the security standards of SuPHP, the user invoking SuPHP must be defined in suphp.conf file and should match the user invoking it. So for a cPanel server, it should be call by the HTTP user which is “nobody”. In our case it was set to “apache” even there is no active “apache” user on a cPanel server. I met this error on a latest cPanel build; so I guess this is a bug with their latest EA3. To correct the issue, I set the value of “webserver_user” to “nobody” in /opt/suphp/etc/suphp.conf and restarted apache service.