I uploaded another person's map and we were trying to make him the author on tmx. He accepted co-author request but didn't know how to claim the map (he does now, just had to link his ubisoft account) so I tried removing myself from the co-author list. When that didn't help, I tried to add myself back but couldn't because "this user is already the author" or something like that. So instead I removed him from co-authors as well and now the page is broken because I assume the author index goes out of range.
The fact that you can even do that seems to me like a bug in the API.

The map in question: https://trackmania.exchange/maps/188076/wikked

Edit: map page works again, I don't know if it fixed itself or someone from the staff did it but thank you either way

I had a similar issue with one of my maps, I appeared as co-author altough I had created it and it showed a delete button to the right of it, so I thought maybe deleting the co-author thing would make me the author since I was the only one with the map, and now the page is showing the same "index out of range" error when I try to enter, so it appears that the bug is still on the site.
Map: https://trackmania.exchange/maps/191465/training-10-long

The page works now. So either we fixed the bug, or we at least fixed it for your map pages.