I ran into this problem when I created a server on a dedicated server. I tried various configurations and even downloaded again to start from scratch. Still had the same problem.
A bug perhaps?
Another Nexer (BattleBabe) told me she also experienced the same problem when running a server locally.
Mirio: you dont by chance have a "teaching0.1_20080113.pk3" file in your data dir do you? If so, try removing it and see what happens. If not, try removing all extra maps from you data dir and run the game. Specifically look for any pk3s that may have a progs.dat (older nexrun/nexball). Then add the maps back a few at a time and see whats breaking it.