Before jumping in, it would be useful if you could clarify some points:
Is this specific to one template? If so, what is that template, how did
you get it, and what changes (if any) have you made to it?
Are you able to start any applications from the Q Menu for the
template? (Try moving applications from the LHS to the RHS in the
Settings.)
In dom0, try to run these commands, replacing TEMPLATE with the name of
your template:
qvm-run -p TEMPLATE qubes-run-terminal
qvm-run -p TEMPLATE xterm
qvm-run -p TEMPLATE uxterm
Start the template and open a console - log in, and try to run xterm,
uxterm.
Report back.
I never presume to speak for the Qubes team.
When I comment in the Forum I speak for myself.