There can be many reasons for an Internal error in MediaWiki. To get more information about the error, you should enable debugging in your MediaWiki.

You can do this by adding the following line at the bottom of  the LocalSettings.php file:

$wgShowExceptionDetails = true;

This will give you the exact error message, as well as a backtrace result so that you can see what is causing the problem and look for a solution.