Did you try deleting the oauth application from Github and recreating it from scratch ?
I think I faced this issue once and recreating the oauth app worked but I have no idea why. If this doesn’t work I have no idea what you could try next, sorry 
Did you try deleting the oauth application from Github and recreating it from scratch ?
I think I faced this issue once and recreating the oauth app worked but I have no idea why. If this doesn’t work I have no idea what you could try next, sorry