UCS has its own (better: additional) methods to work with the repositories.
There is a list of sources in the files in /etc/apt/sources.list.d/.
In some cases during a stuck upgrade I have seen that these lists are incomplete. As a result apt (aptitude, univention-install) may be unable to resolve dependencies. It seems to be a valid workaround to add the missing sources to the sources.list file until the UCS methods are working again.