[CRASH] Ingame Client System Error: Critical Error has occcured (over and over again)

I can't reconnect into the Game anymore. Everytime I press on reconnect, the ingame client starts with blackscreen and I instanly get a "Critical Error" message. I can't Tab or Alt + F4, just end the client programm with Task Manager.

It happened after I started an ARAM. The Game starts, I move and crash. I tried to reconnect but it always said "critical Error" Then i restarted my PC and played on another account just to try. Everything fine in Champselect but as soon as the ingame Client starts "System Error". I uninstalled LoL and even deleted the Folders and after downloading the game again, started a Practice Tool game. Champselect works fine but the ingame Client crashes instantly..

Not even "Initiate Full Repair" helped. I tried this over 10times now on practive tool. I won't try a normal since I don't want to get a penalty for it. It's not even possible to create a Dump FIle.

Edit: The launcher works fine and I have Windows 10