I could reproduce the error message on restart of horde (
univention-app restart horde), but after some seconds it works again.
But since horde is also a server, based on docker, there might be some extra work, if you change the IP address of the host. The UCR value of “nameserver1” should point to your masters IP address, etc.
root@host:~# univention-app shell horde
root@horde-70481211:/# ucr get nameserver1
Instead of searching and fixing all the needed settings manually, it could be a bit faster to uninstall horde in the App Center and reinstall it afterwards.