Mouse in game flips left and right switch around

I just bought a new mouse, for left-handed people. I changed the buttons to the previous settings I had. It works perfect in LoL-client. I start the queue, everything works fine.

Now, the moment ''Game accept/decline'' pops up after finding a game, the buttons are suddenly changed.

So to accept game ----> left-click (normally) on accept Now it changed to clicking right. In champion pick the mouse is back to how it's supposed to be and how I want it. The game loads, I come into the game and it's the otherway around again.

I tried some things with the hotkey-settings, but I just can't figure it out .

Anyone?