Recently I got stuck inside several stones too, maybe same reason.

If you are spawning the vehicles, some random idea could be to spawn without driver (don't know if it is possible) or with something broken so they don't move.

For me works, maybe your router or firewall.

Two days ago I just tested this, the first time failed, then I checked my router and UPNP was disabled. After enabling it, and rehosting the game, it worked and a random Russian joined in.

two years and four months later still unsolved... the UAV FSM must be really messed up.