well, someone uses your cd key
maybe trojaned bot or so?

dunno if it MUST be a ban
try to log onto your acc(cd key) with a bot! (repeatingly)

because maybe someone stole your key
and runs a bot all day long
--> u cant connect!
if bot gets error or so, you can log in!

but there are lts of possibilities, like your still logged onto btlnet for any reason
try to log onto other realm =)