Hello RudolfE,

I just gave it a try using HCE 1.11 and Rebel 16.2; the behavior looks to be very similar :

- the downloaded package includes a book.bin file in a polyglot-books subfolder
- letting the default UCI setting for bookfile = book.bin doesn't work
- copying the book.bin file to the rebel executable directory doesn't work
- assigning the executable Rebel program folder (where book.bin has been previously copyied) to the bookfile setting doesn't work
- but letting the book.bin file in the original polyglot-books subfolder and assigning this complete path to the UCI bookfile setting works (at least for me)!

Therefore, the setting for bookfile should be input like:
C:\your_path_to_RebelEAS\polyglot-books\book.bin
and later displayed as:
file:///C:/your_path_to_RebelEAS/polyglot-books/book.bin

Weird, but it seems you can't remove the "polyglot-books" layer.
Hope this helps,
Tibono