Right Mouse Button not working (M2)

[Nymra]

I accidently assigend the Mouse 2 (M2) to something in the game settings. 

Now I cannot asign it to "Block" or "Left Hand Attack" anymore, because that option does not exist in the settings o_O

The Button still works, I can asign it to "Right Hand Attack", but basically now I am unable to map Block/Left Hand Attack to the M2 anymore

 

Anybody has an idea/fix for that? There also seems to be no way to set the Settings to default...........................