I bought HoT serial key and redeemed it on account.arena.net. It worked well and I got HoT activ at my account pages. Started the game and it worked there too.
Now when I want to play again my HoT have disappeared. Also on arena.net. It is as if it never existed.
EDITED
I try serial code again and got this answer
“The serial code given was not recognized by our system. Please try again.”
Yet I have been playing with it!?