Just change this page in your wiki:
MediaWiki:Mainpage
[[MediaWiki:Mainpage]]
If I go to:
MediaWiki:Mainpage
and click "edit", I just see
Main Page
If I change that text, then the system wants to look for a page with
the next text as the main page.
Please remember, I don't want to change the URL of the main page,
just the displayed text "Main Page".
Tim