I'm having this problem, too. Is Riot even aware of the issue?

Yes, we are aware of the issue about the custom games.

Apart from WintersHeart666, did anyone else get it in a queued game? If so, can you give details of what exactly happened before getting the error?

From what I can tell it happens when someone leaves the game. I don't think it registers properly so it doesn't return to the lobby. Essentially you get stuck in a ghost game.

I had the ''Active game not found.'' issue so i left the champion selection, and now i m stuck with this message ... ''You will not be allowed to join the queue because you are currently assigned to an existing game.'' but if i try to join another custome game it says ''Active game not found.'' and theres nowhere i can join again this ''existing'' game ...

(Almost) Every ARAM attempt ends with this message and the game not starting. Considering it's about 1 in 10 games where someone doesn't leave and about 1 in 10 games it works...probably not a coincidence. Instead of returning the remaining players to the lobby, it's bugging the room/game completely.

All Draft All Mid works fine, since getting your choice of champ and leaving is just silly.

Still broken. Judging by the time of these posts it doesn't appear that anything is being done about it...

sometimes you can check if the game is gonna start by changing your skin/ summon's skills

Is anyone still seeing this after the deploy tonight?

Yes it just happened to me in a custom game (ARAM). It happens when someones leaves.

Works fine for me. :\