Ventura refusing to open ePubs

Recently bit the bullet and "ugraded" from High Sierra to Ventura.   Now I can open ADE, and even see the cover of an ePub from my library, but it refuses to open the book, with a popup with a banner across the top left corner saying "Missing."

I had a similar problem with downloads of torrent/magnet files but managed a workaround by changing the local of their destination (browser downloads were no problem).

So I suspect that if I move all my ePubs to some new location, it might work.  But where?

Any advice or suggestions very welcome!