how to solve forbidden
You don't have permission to access /modules/Messenger/messenger_postProcess.php on this server.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.
403 Forbidden errors are commonly caused by the file permissions on your server. For most server setups, the files in your Gibbon root should be 0755 for folders and 0644 for files. If you run the permissions change recursivly (often through the command line or a tool like Filezilla that has a 'recurse into subdirectories' option), this usually catches any files that need their permissions set.
If that doesn't help, be sure to check your PHP error logs to see if there's any other clues for what might be causing this.