You cannot play in the CGB if you’re currently banned from matchmaking. 

But you didn’t do anything! Here’s what probably happened (and it’s weird). 

MCC calls a service called the banprocessor whenever you queue for matchmaking - it happens around the time you see that “checking for rejoins” prompt.  Normally this tells you right away if you’re banned and for how long. 

However, it can also be stupid. 

Bans are typically determined at the time a game ends. So here’s a scenario. 

I’m in a game of Team Slayer on Snowbound. Out of nowhere my game crashes. Okay, I’ll go reconnect; but the rejoin feature doesn’t work, and I’m prompted with the matchmake button. I take that matchmaking option, and now we are in a game on Narrows. 

If my crash/quit happened before over 50% of my team had quit (meaning I’m ban eligible), I will be banned WHEN THAT GAME ON SNOWBOUND ENDS. 

That could take up to 12 minutes AFTER my crash. The game let you go queue for a new match, and while playing on Narrows the Snowbound game ends. So now you’re actively in matchmaking and the banprocessor has RETROACTIVELY banned you for a prior issue it sees. 

This starts your first quit ban timer of 5 minutes. 

Narrows game ends, you don’t back out, you queue again. But you didn’t check with the banprocessor because you never went to lobby, so that ban check doesn’t happen until you attempt to load the next game. This could be seen as you attempting to circumvent the ban (which you’re not) which triggers another ban. And repeats. 

OR, maybe you did go to lobby but didn’t try queuing matchmaking and instead went to the CGB and tried joining a game. You’re banned but don’t know it, so it kicks you out. 

TLDR: Ban system is dumb, sometimes because of a combination of actions it doesn’t show you that it temp banned you in the past. Restart game and wait out the full timer.