registration of reserved mail address for icloud failed

Apple reserved a mail address for me in icloud. The mail box works, but I can not register the mail address in icloud, nor on the ipad, nor on my mac. System is saying that the mail address is already used by another apple id. Also following the instructions to register the mail address as indicated in the mail to me by apple, on my ipad and mac fails. What am I doing wrong?

Thank you in advanced for your help.