Dmenu can't spawn applications in qubes

When I press mod+d and select “sys-net: Terminal” in order to open a terminal window within the sys-net qube, nothing happens. In fact, dmenu can’t spawn Firefox in any other qubes, too.

Where can I start debugging this?

Edit: I can open a terminal via dmenu in the dom0. But any other qube is unresponsive to dmenu app start selections.

Old thread. I didn’t have such bug persistently. I guess that was one-off. Restarting the system fixed it.