Invalid format while the format IS valid in the options

So today, right after the update of the patch 7.1 i went into a custom game to check how syndra was doing with the new W, and i got hit with a bad surprise. As soon as the transition from the client happened, my screen went black and a good old "INVALID FORMAT" started to dance around on it.

Needless to say, this never happened before this patch and after i checked around and discovered i could check the resolution of the game in the file, i quote : "Game.cfg" which is located in "\game\data\cfg" within the LoL folder, i discovered with surprise that the resolution did not change (i play 1024x1280) .

So, what is going on? Am i the poor little victim of a big bad bug? (And yeah, since the screen go black i can't take any screenshots. As for what i did, i simply launched a custom game like anyone would, tried twice with and without bots, did the same thing twice. I am using the new client, and possess a TV as my screen.)