Blurb Export hangs

I created a 99-page book in Lightroom Classic 12.3 Book module (no updates available). I made a similar book last November and exported without a problem. And I've done other books in the past. Blurb hangs about halfway through the export. I get no error messages. It just stops working. At least one of the times the bar at the top left said 53%, but most of the time, it just shows a progress bar, with no message other than exporting, as if it is still working on it. I've left it running overnight and it never gets any further. I am able to export a PDF of the book.

I've deleted the cover photos since I know Blurb exports the cover separately when exporting a pdf, but the Blurb export still hangs about halfway through. The only unusual behavior this time is that I get a message

"Photos with transparency"

Some photos in the book have transparency. The transparent areas will show as opaque in your book", and it lists quite a few pages. I've never gotten this message before but I've also found threads that say it isn't really a problem, so I don't think this is why the book isn't exporting. But as a separate issue, Please advise if I should be fixing the photos with transparency before printing.

My computer specs:

Dell XPS 8930

Processor Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz 3.70 GHz

16 GB Ram, 15.8 GB usable, I ran a pre-boot scan and found no errors.

Windows 11 Pro Version 22H2 (no updates available)

OS Build 22621.1778. 

Thank you. Bonnie Giddens