Manual:$wgSend404Code

Details
Web servers are supposed to send 404 responses for any not found page. If someone tries to load an article that doesn't exist, MediaWiki sends a 404 status. (Note, editing an article that doesn't exist does not send a 404 status) Some webservers try to intercept 404 responses and give "helpful" 404 response pages, which is quite annoying. Some webservers get the 404 intercepts severely wrong, causing garbage to be output. In either case, you should consider looking for a new web host, but if that's not an option then this setting can work around the issue.