Fixed, now 
For others who might find this thread:
- check your overall setup and installation
see: Debian-10-minimal Configuration - #94 by Sven
and the official notes/apt-cacher-ng.md at master · unman/notes · GitHub - check your
qubes-core-agent-dom0-updatesinstallation with:
dpkg-query -l qubes-core-agent-dom0-updates - check your dom0 update settings:
→ Q → Qubes Tools → Qubes Global Settings → qube defaults
→ Dom0 update qube: apt-cacher (or if you use another appVM i.e. firewall)
In my case the issue was that the Dom0 update qube setting was pointing to my firewall appVM instead of my apt-cacher appVM.