Often this means that the server found a problem with the permissions on your script.
For security reasons the web server performs a couple of security checks on scripts before they are being executed.
In most cases you want to check the scripts permissions and see whether
- it is set executeable
- it is not writeable by anyone else than the owner (not even the group which is the default)
In your log directory you will find the suexec.log which contains an explanation of the problem. Note that the suexec.log gets updated every couple of minutes so you might have to wait before you can actually see the log line explaining the problem.