Thank you @tetris4 and @daniele_athome for the fast respond.
I tried to register again and the first try ended with the same error message.
The second try gave me the “retry in a few minutes” error message, wich must be from the new spam countermeasures. (So, I can verify, it works )
Maybe I have another issue…
The app always shows me a wrong phone number in the first place, something like 017600000000.
It seems like my phone identifies it by this number.
Some Googles searches showed, that this must be the number saved in my SIM by default (It is not my first SIM, I ordered a new smaler one for this phone).
Can this lead to a verification error? I always entered the correct number before I tried to register.
I can not change the number on my SIM as far as I now. (Not without a SIM reader, or older phone).
As mentioned before by someone else in this forum, a register website for the desktop client would be great.