Well, problem solved now. Just to collect some knowledge i will tell, what happened:

I paid for my account via Paypal and 3 or 4 days later i got this message "login failed!"
I waited some hours, but the problem remained, so i deleted my account and tried to reregister, but no confirmation SMS arrived.
After waiting more than 30 minutes i tried the button "voice registration" and some other stuff and suddenly i got the message "failed: reason blocked" (or similar).

I tried to register on my Android-tablet, but it also told me, my number was blocked.
So i emailed Whatsapp and asked them to unblock my number, what they did 4 days later (weekend was in between).
I tried to register again and waited for more than 2 hours for the SMS and then tried "voice registration" again... and was blocked again.

So i emailed them once again and they unblocked me again and now i managed to reregister by sending the registration request from the Android pad and the SIM in my N9.


So, long story short:
Using voice request seems to block your number, so i would not recommend to use it anymore.
sorry, i don't want to make further tests on this for confirmation

And for future users who have to pay for their account:
Don't panic, when your account does not work for some time after paying.
Later i heard from Android-users, they also had no service for about 24h, while their account was transferred from free to paid, so this seems to be normal... i guess i just should have been waiting longer and do nothing?

Maybe anyone who has paid for his account recently wants to share his experience?

// Edit:
the strangest thing is: i'm now succesfully registered again, although i still received no SMS!
so, i'm pretty sure, they changed something at the registration process again.

// Edit 2: Scorpius fixed it
http://talk.maemo.org/showthread.php?t=84605&page=334