I installed the Whonix 15 templates via
sudo qubes-dom0-update --action=install qubes-template-whonix-gw-15 qubes-template-whonix-ws-15
and removed Whonix 14:
sudo rpm -e qubes-template-whonix-gw-14-4.0.1-201811040215
sudo rpm -e qubes-template-whonix-ws-14-4.0.1-201811040215
I replaced 14 with 15 in:
/srv/formulas/base/virtual-machines-formula/qvm/whonix.jinja
as described in:
and I rebooted dom0 as described in:
https://www.whonix.org/wiki/Qubes/Install#Footnotes
but the error persists:
sudo qubesctl state.cls qvm.anon-whonix
'state.cls' is not available.
DOM0 configuration failed, not continuing