Not sure about how to reproduce it, but the first event was **URF Deathmatch **and the second one was **Push the Cart**.
URF event worked normally, nothing wrong happened.
On the second event we pushed the cart to the tower, it exploded but we didn't received a reward. The game got broken after that. (the bugs everyone been reporting lately: Rift Herald stuck, some abilities dealing no dmg or weren't working, other had 0 cooldown, etc)