If you have a map hidden and try to upload a replay, you are redirected to the main page of MX.
If you unhide the map, you can upload the replay as usual. If you hide it again, the redirect returns.

Well this is logic

Not at all. Imagine, the following.
You are building a map for a competition, and you prepare everything of the map while it's hidden. So on the day the map is released for the public in the league, you just need to unhide it.

That was a joke

This is 'fixed' now, thanks .