Sorry, didn’t answer this question adequately.
The Whonix appVM that fails to start is based on the whonix-ws template as are other whonix appVMs that continue to function correctly.
The DVM is (was) based on the same Whonix-ws template.
Something else interesting that I just noticed is that in Qubes VM manager for the failed DVM, VM settings/advanced shows:
Initial memory:3412MB
Max memory: 4000MB
VCPUs no. 1
I did not change any defaults when the DVM was created.