I am getting this error which others have gotten, However my setup is using IPv6 only, a fresh install of univention 5.
There are no issues with DNS resolution and the firewall rules are only the defaults that work with UCS5, I am wondering if maybe the activation server is ipv4 only maybe ? I am not seeing any other errors.
This seems to be an issue with IPv6 only configurations, because I installed a new server with UCS 4.4, and am getting the same issue.
This issue does not occur when I set ipv4 addresses on these hosts. But due to ipv4 availiblity issues, really feel that this should work with IPv6 only configurations.