It turns out that QB sometimes opens a Windows Security Warning and ask to click 'yes' or 'no.' This is when the Quickbooks "freezes."
I've noticed this prompt before which seems to pop up here an there randomly. If they can get to the dialog box and close it, and also the larger "Quickbooks" window behind it (see screenshot below) then they can continue their work.
But if the dialog box opens behind the frozen screen and you can't get to them, then they need to use the workaround of force quitting QB. They know how to use the task manager to force quit now so they have a workaround until we resolve the issue.