E.g., RequiredUCSVersion does nothing if only set on the latest version. When upgrading, RequiredUCSVersion from the installed version is taken.
Fixed in univention-appcenter 5.0.21-7.170.201606131112
The actual error message is a bit weired, but it works 2 errors occurred: (must_have_fitting_ucs_version) Unable to upgrade dudle. Aborting... OK - univention-appcenter.yaml
<http://errata.software-univention.de/ucs/4.1/206.html>