After searching the forum for recent relevant issues, I found these topics that are about the same problem as my OP:
One thing with the first link I share here is this:
@alimirjamali I am seeing update availability nofitier in my xfce4 system tray. However, I am still getting no GUI pop-up when I click on the update availability notifier. Please check my OP for details.
In this case, what should I do? You say the If the update availability finds updates to dom0 (or other templates) the update GUI should work when it is clicked on. But in my case, I have the update availability notifier visible on the tray, yet no updater GUI pops-up into action when I click on the updates.