So you have seen it in Vikingr too? And mods are not causing it... this seems to be a really random bug in multiplayer of M&B:WB. I guess one can play long time without problems - I haven't had that crash for days now, but when I got it, it's usually after I die. It lags really long, and it might "settle down" after a minute or two. When the next round starts, it happens again so it might be related to scenes where are lot of people spawning or where battling is going on - scenes that cause spikes in framerate, like beginning of the round, and death.
In brief, just an another form of crash.