The "usemod" interwiki prefix must be updated from http://www.usemod.com/cgi-bin/wiki.pl?$1 to https://www.usemod.org/cgi-bin/wiki.pl?$1 It appears that the former URL was acquired by someone whom we do not want to link to. Originally reported in this spot.
Topic on Project:Village Pump
m:Talk:Interwiki map is the correct venue for this. I'll handle it in a few days when my meta RfA passes if nobody else does.
Thanks a lot for the pointer. I was already wondering who may be permitted to change the interwiki. I just added the request at m:Talk:Interwiki_map#usemod as a service for you to work on once you have the required permissions.