If you are banned from mw3 would it even connect to multi player? (I would think it would not let you connect to a server period if banned...however I am not banned so not sure.)
But m0difi3dr34l1ty asked his question because when someone put a carriage return (enter key) in their clan tag, the same thing would occur. Try taking off the clan tag real quick and see if that fixes it? Just to see.