Hi Igor,
the Keyboard Shortcut Ctrl 2-[Numeric Block] or Ctrl 8-[Numeric Block] which makes cursor jump to the next/previous folder in preview window works only when NumLock is OFF, so it should actually be the up and down keys on numblock unless you change it (which IMHO is probably best).
But I also noticed that the same functionality exists by using Ctrl-Up and Crl-Down (not on the Num Block) but in the right click menu (which BTW appears only when cursor is on folder, not on file, even tough the functionality is still there - maybe you would like to make this appear also) it only shows the more awkward Ctrl 2-[Numeric Block] or Ctrl 8-[Numeric Block] shortcuts. Ctrl-Up and Crl-Down are also not mentioned in the help/shortcuts list either.
BTW it took me a long time to find out that these (and some other neat shortcuts listed in Help > Key List) actually exist. I think it would be more user friendly to have all functions listed in the menus (probably view or tools), since most (new?) users will search there for features.
Kind regards,
Brahman