In the bookmark pane, the "+" button is disabled / greyed out. In the "reading" menu, the "add a bookmark" menu item is there, but using this will freeze the software. When frozen, the "exit" (x) button in the upper right corner works, but nothing else will (no response to any mouse click or any keyboard button).
I will reply to myself here - a book has to be added to the "library". After that, the book mark feature works.
Still, for books that are not added to the library, the book mark feature should not freeze the software.