Did you upgrade Qubes dom0 beforehand?
Are you using Qubes R4?
Not required but also not hurt.
Not required but also not hurt.
Meaning that your dom0 was not up to date.
Not easy to know for sure, however this is a Qubes rather than Whonix issue (as per What to post in this Qubes-Whonix forum and what not.) therefore should be reported at Issues · QubesOS/qubes-issues · GitHub.
Whonix maintains the inside of the templates but the outside of the templates, i.e. everything related to qubes-dom0-update, qubesctl, salt is up to Qubes.