When creating a local repository on an UCS 5.2 system with univention-repository-create the default for the starting version (repository/mirror/version/start) is 5.2-0, so only this version will be synced.
If you want to update systems from 5.0-x to 5.2-x with this local repository, you also need sync the intermediate update 5.1-0 to your local repository.
You can do that by running the followings commands on your repository server:
$ ucr set repository/mirror/version/start='5.0-9'
$ univention-repository-update net