A quick update:
After some desperate searching through the forum and trial-and-error, this finally helped to resolve my issue.
To spell it out explicitely, the solution (for qubes-gui-dom0 version 4.1.17) for was to run the following in dom0:
sudo systemd enable qubes-qrexec-policy-daemon
sudo systemd start qubes-qrexec-policy-daemon
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