The redis-server was just another issue, that had added complexity to the failure of rebuilding. It was kind of a loop: The external redis configuration had changed, while the pre-rebuild app state needed that specific redis-config to start. It couldn’t rebuild though, as the theme component fetching didn’t work.
I was lucky though after 20-20 rebuild runs and finally the theme components updates got fetched.
In the overall app design context, it would be nice to have documentation on how to rebuild in a “safe mode”, i.e. rebuilding the app independently from plugins or themes. I couldn’t find a hook to process the theme components, or how to deactivate (in contrast do uninstall) plugins either, which was a bummer.
Edit: Whoa, now I see the link to the safe mode. I couldn’t find that before (no Google for us in China, try to find anything relevant with Bing…). Gosh, that would have helped a lot!