The only time I've encountered 500 errors that I recall are when there are problems with folder permissions with Tiki itself.
Perhaps the documentation on this page will help you verify the permissions. In particular, pay close attention to the
chmod commands, which lists the particular folders that need special permissions beyond the standard. Even if you can't use SSH to run command line commands on your web host, you can still set permissions using an FTP client like Filezilla. I'm not sure if any of these folders would be related to your problem, perhaps the template folders? It certainly wouldn't hurt to check and eliminate any kind of permissions problem.
Clear your Tiki cache too!