The update to another UCS version is blocked if a repository is configured and this repository does not exist on the next version. This is pure updater logic. We happily used this feature to test whether the installed Apps are available on the next version. Unfortunately, this does not work for Docker Apps as no repository is set up (and no repository exists specifically for the next version...). The updater should ask the App Center whether he may update with all the Apps installed. The App Center in turn should take care of all the "update" / "no update" checks regarding Apps. I think the App Center should also administrate the repositories for the Non-Docker Apps, but this is optional.
Fixed in preup.sh
Please reopen if necessary