Than it could be another problem. You can have a look at https://www.mediawiki.org/wiki/Manual:How_to_debug to find a way to get more information about this error as the currently shown HTTP 500
I did it and I have the following message :
Fatal error: Cannot redeclare class SpecialMultiUpload in /data/web/6c/03/7c/grondin.tuxfamily.org/htdocs/extensions/MultiUpload/SpecialMultiUpload.php on line 120
So, I deactivated the extension and it runs.
Thanks for you support.