The Thumnail from the game isnt working for the moment.
Announcement in Trackmania discord:
"If you encounter black map thumbnails after saving a map or other weird issues with the mediatracker inside it, it's probably because your Shader quality setting is set to Low/Very Low. As a workaround for now, you can set Shader quality to High/Very high.
The fix will probably be deployed after the weekend. Thank you for your patience"