Qt WebEngine Render process crash

I'm receiving this error message in the edit of an EPUB in Calibre v6.17:

"The Qt WebEngine Render process has crashed so Preview/LiveCSS will not work. You should try restarting the editor."

This seems to be a problem only for one book and restarting the editor did not resolve the problem.

The preview pane reports "Loading preview, please wait."

LiveCSS reports: No style information found. Move the cursor inside an HTML tag, even though it is.

Are there any other trouble shooting steps I could try before throwing this edit away and starting again? I'm deep into this job, so I'd rather not start again.