When i start my game as Listen Server, everything works fine as long as i control the server. As soon as i control the client, i can move for a short time and at some point the client seems to lose sync and the characters run straight in the direction they went last. Meanwhile on the server everything is still fine and the characters move where the client is clicking:
Try setting the Always Relevant flag in your actors.
If this works, then uncheck the flag and increase the Net Cull Distance Squared value (X2-X3 or more).
Most likely your actors are too far from the player’s camera and they are being discarded to optimize network traffic.