Everyone having this problem... I DID find a sort of buggy fix...
So after it didn't let me sign in 8 times, I ran the "Hextech Repair Tool" as administrator, and it crashed midway through scanning...
Then I screwed with the settings for a while and eventually it crashed again.. and again. and again. Lot of crashing.
Then I started League up as administrator and it ran fine, I got into the main menu and then it brought up the error message again.
THEN I tried the cycle again and this time I queued up for a game quick enough to avoid the error and have been playing nonstop since <3