I have seen similar problems with R3.
Have you tried just stepping through after the error?
In my case, when I did this Qubes started and loaded the manager, and
all vms had been created. The netvm would not start. I removed one of
the network interfaces and that solved the problem.
No disposablevm showed in the manager. However, when I tried to use a
dispvm it was automagically created, (after the normal "wait while
updated" message.)
unman