I noticed a pattern ...
I have 4 accounts for WoWp (EU, NA, Asia, CIS) and all 4 are set up in the Gaming Center. Sometimes I log in with one of those accounts in WGC to make that my default server for some time. After that, in game I use 'disconnect from server', change the server and log in (connect) again to play on another server. Now I noticed it is ALWAYS the default server password that is forgotten !! I can disconnect and connect to 3 servers without a problem, but only the default server very frequently claims that the password is incorrect.