You might be running into this (hopefully temporary) issue: dynDNS service down? - #3 by peter
Also using something like example.com as your subdomain may not be a good idea. (I don’t know whether deSEC allows this.) Select a subdomain without any periods in the name. You can create entries in that domain later. So in your example you would create com.dedyn.io, then create entries for example in that zone.
Note that deSEC e.V. is not a domain registrar. The purpose of the dedyn.io subdomains is DNSSEC secured DDNS. It sounds like that is what you intend to to use the subdomain for. But if not, you need to register a domain name elsewhere.
fiwswe