I guess I put this is the wrong thread. Others have had this problem:
here. After reading there I tried registering again putting the OTP (from phone) directly after the password (no spaces). This worked. However, although I read here somewhere that the old Kindles would not download the new DRM (2023), this is not true. I can download read the new book to the old and new Kindle (both registered to me), but neither "version" is viewable in Calibre after uploading and bring the message "This book is locked by DRM."