Password problem

I logged in to Memecenter using Facebook. Now whenever i enter Memecenter a window pops up which asks me to complete my account and enter a new password twice. No metter what i enter, it always shows the error "Please make sure two passwords match each other and you are not using an already taken username." I'm pretty sure my username isn't taken, and the passwords i typed were both correct, yet it never accepts them. That window which pops up is very annoying and there is no other way to get rid of it.