LAN game bug?

Hi,

When I try to play a LAN game with my friend the following happen:
In the begining we can see others movement and progress, but after a 5m we can keep playing but for some reason we plat in "own" maps.
On my screen his movement and progress are stopped and otherwise.

We can still chat and give stuff to ourselfs, but were playing on diffrent maps. (in my game I can't see his progress and the enemy wipes him out, when he is still able to play on his own pc.)

How do I solve this problem?

Sorry for Bad English. I speak dutch.