Thread:Project:Support desk/sidebar is deleted

I want to edit the sidebar of my wiki. The URL to the sidebar  shows a page that tells me, that this page has been deleted. Of course the sidebar is not deleted.

Why I get this message? How can I edit the sidebar?

Version information: MediaWiki 1.22.1 PHP       5.4.20 MariaDB   5.5.33

Thank you for help. Ferdinand