That resolved the System Update issue in the GUI, but left RestApps, UCP Daemon, and Xmpp Daemon not running.
fwconsole start xmpp
fwconsole start restapps
Started those two, but fwconsole start ucpnode wouldn’t start UCP.
I had to re-download it (fwconsole ma downloadinstall ucp) and then fwconsole start ucpnode to get it running again.
So for this install the issue seems to be resolved, and I wonder if there is still a problem somewhere with the code that caused it in the first place.