horde currently uses ucs-appbox-amd64 UCS 4.3-3. For UCS 5.0 is must use an UCS 4.4-6 basis. +++ This bug was initially created as a clone of Bug #51625 +++
Horde will not be available on UCS 5.0. See https://www.univention.com/blog-en/2020/12/ucs-5-0-discontinued-features/ I close the bug with won't fix.
Please reopen if necessary