I finally have my account back (the usual: account address changed – not by me).
Initial ticket was from Sept. 3rd while I was on holiday, which I closed on Sept. 7th due to misleading subject and made a new one (new subject: Hacked Account). In this I provided the usual stuff (Display Name, Serial, Pre-Order Nr., even attached confirmation mails from ArenaNet about pre-order and registration).
Thanks again for your help, ArenaNet.