League Client disappearing after starting the game.

[CRASH] [CLIENT] So I'm having issues starting the game since yesterday. It really came out of nowhere... Every time I start the game, the League of Legends icon pops up and the loading client for about 2 to 6 seconds, then it just disappears as if I never opened it. There is nothing left in the task manager and not even an error popping up.

I've tried many different things, but nothing really helped. Here is a list of what I already tried:

-Reinstalling the game -Turning Firewalls off -Deactivating all my Anti-Virus-Programs -Deleting different folders, such as "Riot Client" and then running it again (this sometimes lead to me being in the client for a couple more seconds) -Running as Administer (I did this basically every time) -Reinstalling with a Hextech Repair Tool -Basically closing every single task in the Task-Manager I could, to make sure nothing is causing the problem in the background. -Updating my pc to the newest windows version. -I did a "Clean Boot" where I also rebooted my pc -Switched to a different Wifi -Had a friend logging in on my ACC to see if this had something to do with my ACC _

And some small stuff I can't remember... But nothing really helped. As said in my list, sometime I got a bit further, where I could see my client and click on my profile for instance, but I only had max 10 seconds... afterward, it would just disappear again.

It seems to me like a very simple problem but I couldn't find a solution, even on the internet. I also just found report bugs that were kind of similar to my problem but not really. I would appreciate every help or solution you could give me :)