Help! "An unknown DirectX error"

Hello!

I have already played during this patch, but since yesterday whenever I go into a game, I get "Error : An unkown DirectX error has occured and League of Legends cannot start. Please make sure your video card is using the latest video drivers from the manifacturers".

I have played 3 days ago and before, worked just fine. Two days ago, I did try to add "ja_JP" to the available languages for EUW in system.yaml as one guy mentionned in a reddit post (so as to test Japanese voices). When I tried to launch the client, it crashed, then next time I launched it it went "back to an older version of League of Legends", downloaded 1.3 Gb of data, then I was able to use the client again. I thought no way something as stupid as adding a single line (- ja_JP) in one file could make it do that. I tried again, and sure enough, adding a single line to put another option in the languages for EUW makes it download 1.3 Gb of data to try to fix it.

Anyway, I then gave up on the idea because I don't really care, I just wanted to test because people were saying it works really well (on one reddit post). But since then, I tried to play yesterday, client works fine, I can talk with anyone, etc. but whenever a game starts, I get the "Unknown DirectX" error.

I have tried to delete the registers in regedit and download a new version, but I already have the latest version of DirectX according to my PC. I have DirectX 11 with Windows 8.1. I have tried restarting my computer, deleting registers and restarting, downloading whatever I find about DirectX but I already have the latest DirectX anyway... I have no idea how to delete DirectX but it seems like a bad idea. And lastly, I have just tried uninstalling and reinstalling League of Legends (damn it became so heavy these past few years!). Still doesn't work.

Anyone has ever got this before? Please help ;-;