Had been using Whonix GW Testing on Qubes OS 4.0 RC2 without any issue.
Following Whonix GW Testing update, sys-whonix would no longer connect or bootstrap.
Tested on another PC with same result.
Reinstalled Qubes OS and the default sys-whonix proxy based on the whonix-gw Stable template worked fine.
I upgraded this template, rebooted it and the sys-whonix proxy, and they continue to work fine. Tor version is Tor 0.2.9.10 (unrecommended).
Cloned whonix-gw Stable template and set it to Stable Proposed Updates repo and upgraded, rebooted, etc.
Cloned whonix-gw Stable template and set it to Testing repo and upgraded, rebooted, etc.
Created a sys-whonix proxy based on Stable Proposed Updates template.
Created a Testers sys-whonix proxy based on Stable Proposed Updates template.
The sys-whonix proxies based on the Stable Proposed Updates template and the Testers templates will not bootstrap or connect to tor network.
The two sys-whonix proxies that will not bootstrap or connect are both using the newer Tor 0.3+.