Well. I'm in the dark here.
The editor can't load a map.
Maybe there is something with the Maps.dat file or maybe you gave it a map in a wrong format. Check if it's about 300kb in size.
As I mentioned, worst-case scenario - you'll need to redo your maps by hand. You can just delete everything in Maps.dat. Everything except maps is the same, so it's just them that might give you headaches.