Preview failure

[Glennfrancis]

Running Sigil 2.3.0 on an old, but up to date Windows 10 PC, and when I try to open the Sigil User Guide (for example; I assume this will run in Sigil…seems like a reasonable assumption/test case) the Preview window will very briefly display an item, like the cover, when the Update Preview Window button is pressed, but only for a single second. Same goes for any other content from the User Guide: appears for a second in the Preview window, and then vanishes.
Any other doc I’ve loaded (like the book I’m working on, which displays a TOC just fine) I have no preview at all of. I’ve searched high and low for a clue about what’s going on, but no joy. Am I simply doing something dumb that’s causing this?