Basically I died, apparently from a Proxy mine that was above me (on Underground, I was under one of the side high up platforms when I died), and after waiting for the respawn timer I couldn’t deploy or send any messages in chat. I tried to leave and rejoin the game but I got the “Unexpected error” message and then was thrown back to the main menu, without any matchmaking penalty.
Can’t find any way to properly reproduce this, not sure if it was just a random chance bug or if dying from something above me helped cause it.