When two players using Plagueburst at the same time or Plagueburst + Winter's Grap (spam) at the same time on multiple mobs, it slowly drops the FPS of the hosted game until its crashes.
Before someone says "its because your computer isn't strong enough". I can guarantee that is not the case here :)

The server that hosts the game cannot follow up with all the animations of the spells and therefore end up crashing. (That is my best guess)
Reason why i believe that, is because the client side works fine. Your character moves fine and spell animations are used fine. But every mobs lags tremendously and damage are not being registered till 5-20 sec after using the spell until game crashes.
​
​
This actually makes the game unplayable for my two friends and I, because only one of us are able play the build we actually want. I sure hope the servers will get optimized soon enough.