I have similar problem with game.
Sometimes i just get DC and there is no way to connect back. You get DC then you log in to steam/ Mercury servers and there is information that Match is ready-> loading screen and "Could not join the mission".

I tried restarting game client/Steam but none of those worked.

It's quite boring to wait X min just because you got DC for a sec and cant join again due to some bug or problem related to logging into game?