fluxbox-generate_menu expect that valknut is not named valknut but Valknut and therefore does not include them into menu
Created attachment 94391 [details, diff] simple solution of my problem
Looks like a fluxbox bug to me. And, the patch is reversed, or which one does fluxbox expect actually? Your description doesn't match the patch.
Valknut - is not proper valknut - is proper patch is not reversed, sory for my english. Yes, this is probably fluxbox bug. In future i fill them into fluxbox bugzilla.