Then I must have messed up the registration form somehow...

The account was created but the serial not transfered. So I was not able to use the online-update functionality afterwards.
But as the updater is verbose enough and states that online updates are for registered users only I had a hint of what to do and was able to sort this out with amyrens' help.
As it comes to my mind now... another small glitch in the registration form:
The entered password should always be shown. Only that way I was able to figure out that seemingly no special characters like '%'", '!', ';', '$' are allowed in the password. I use them often for my passwords. If the registration forms entered password is only shown as dots one would not know that special characters are simply skipped when entered and do not end up in the desired password at all - without any kind of feedback that this does happen.