I found the problem!
I was using a Reshade. I thought I removed it for troubleshooting purposes, but as it seems I forgot the one or other file which was in the XP11 folder.
Worked fine after removing every last file of the Reshade.
Since yesterday my in game local map (the stock X-Plane map on 'M' keybinding) doesnt show anything.
Whereas it opens as it should, it only displays the interface with the selectors on the right hand side,
not showing a map, the compass rose, my position or anything. All I see is the blank, blue background of the X-Plane 11 interface.
This happens at any location of the world, using custom scenery or not.
I have not modified the interface with any plugins and tried to deactivate any custom plugins without any success.
UPDATE: Same problem for Airport overview when starting a new flight.
UPDATE #2: Weather is being displayed correctly, compass rose, wind and everything else is not.
Hope someone can help! :)