Slideshow not working--stuck on Preparing slides--even 2

Slideshow suddenly stopped working. It gets stuck on "Preparing Slides." LR Classic 11.3.1.

 I tried it today on just 3 Fuji Raw files and it hung up.  

I have been using it for several years.

The problem is on my Imac 2017 running Monterey, 64GB memory.

It happened suddenly, before Monterey while on Big Sur.

However, it does work on my new macbook pro, M1 Monterey.

I do most work on the iMac & miss slideshow.

Mostly I give up and post to Smugmug for slideshow, but decided I'd reach out today.

Any clues welcome!