There is no way to diagnose the problem without seeing the error. By default errors are not shown because it gives potential information to hackers.
You can temporarily enable it to show errors by doing what it says, edit Web.config and set <customErrors mode="Off"/>
Its possible the errors are being logged in the mojoportal log (assuming file system permissions are configured correctly) at /Data/currentlog.config, if that file exists you can download it and see what errors are being logged. The newest/most important errors are at the bottom of the file.
Once you have the error information I should be able to help.