lilypond-book on Windows 10 always shows usage info

Having installed LilyPond 2.20.0-1 on Windows 10 and setting the path to C:\Program Files (x86)\Lilypond\usr\bin I observe the following using the minimal example from the documentation:

• Invoking the command lilypond-book --output=out --pdf lilybook.lytex a popup opens asking for the program which I would like to use to open lilypond-book.py
• I select C:\Program Files (x86)\Lilypond\usr\bin\python.exe
• Then, only the usage info is shown. Nothing is compiled.
• Even if I just type lilypond-book --version the full usage info is shown instead of the version number.

When I directly call the python interpreter C:\Program Files (x86)\Lilypond\usr\bin\python.exe lilypond-book.py --output=out --pdf lilybook.lytex it works well.

Does anyone have an idea how to solve the problem? I heavily used lilypond-book several years ago on Windows 7 and it performed perfectly...