While all my bookmarks are accessible via the Bookmarks menu just fine, Bookmarks → Open Bookmarks Window displays a completely empty window.
I just learned that this does work for other people. So I’ll have to look into why it doesn’t for me.
I remembered that I reported a bug about bookmarks back in September 2018. The current build at the time was v630.0.318799.
The OW 6 builds for the last few days […] have a data-destructive bug affecting the bookmark files. All
the “a” tags in Bookmarks.html and Favorites.html have had their href
attributes removed by the affected OW versions. This of course means
that the bookmarks don’t work in the browser.
Could be related to your problem if you ran at least one of the affected builds?