In your error log:
Code:
[Sun Sep 28 19:13:55 2008] [error] (13)Permission denied: Apache::SubProcess
exec of /opt/users/sl/d/e/......./php/php failed
732490 [Sun Sep 28 19:13:57 2008] [error] [U==.......] [client 70.190.73
26] (13)Permission denied: getpwuid: invalid userid 732490
Check that all your files have a valid user & group. Contact your host if you can not resolve this problem.
As already mentioned in your ticket, this could be due to mod_security and your host does not allow turning it off in a .htaccess file. You should ask your host if mod_security is installed and how to disable it for your site.