I tried several multiplayer scenarios with a friend recently and on several occasions one of us couldn't get out of vehicles after entering. We would be able to choose 'Get out' from the action menu but nothing happens. All other actions like switching seats won't work either.
Strange thing is: it happens with mods or without, on user made and BIS missions (IIRC in 'Escape from Altis' i was able to exit some cars but not able to drive them, though). It usually won't happen with every vehicle in the mission. And it always only happens to one of us and it doesn't matter who is hosting.
Everything else works fine. We verified our game installation in steam.