Thank you Andrew. But since their "solution" seems to use Fedora instead
of Debian, one could also suggest re-installing qubes first to see if
the error persists, or changing the computer -- I exaggerate to
illustrate that this can not even be called a workaround :-))
Anyways. Whonix still allows manual update in terminal, maybe the
qubes-updater is overestimated anyways ... there were some nice scripts
under 3.2 for automatically updating all templates. Just have to find
hem again. Bernhard