I'm not sure if it's a problem with pihole or if it's a configuration issue with your myfritz/dyndns.
Are you able to access the lists manually, so to rule out that it's releated to pihole?
Your question might be better suited in a Fritz!Box forum.
There are other options to share/maintain lists, maybe this would be something for your case?
https://github.com/stevejenkins/pihole-cloudsync