Files are nerfed somehow, remedied by deleting all files in Steam\steamapps\common\ManiaPlanet_TMStadium\Packs then revalidating the game in Steam Properties menu.
I had that happen after the last update, I launched the game from the Canyon shortcut instead of Stadium's shortcut in Steam and it fixed it for all titles
EDIT - It did the same today, there's an update called maniaplanetlive.zip which is triggering this issue with Stadium. Seemingly they broke Stadium Launcher in the last MP4 update < sarcastic shock>
I had that happen after the last update, I launched the game from the Canyon shortcut instead of Stadium's shortcut in Steam and it fixed it for all titles
EDIT - It did the same today, there's an update called maniaplanetlive.zip which is triggering this issue with Stadium. Seemingly they broke Stadium Launcher in the last MP4 update <
sarcastic shock>
NRL Steam Group - https://steamcommunity.com/groups/TMNRL
TrackMania United Forever Garage - https://bit.ly/2TLOT3D
ManiaPlanet Garage - https://bit.ly/3rJmMyN