It seems for some time (at least Far 3.0 build 4545 till 4700) has broken User Menu navigation. Help says :
As Shift-F2 works OK, the BS works the same as Shift-F2, and I am not able to switch to parent folder User Menu.The local menu may be placed in any folder. You may switch between the main menu and the user
menu by pressing Shift-F2. Also you may call the user menu of the parent folder by pressing BS.
Can anyone confirm that it's not only my configuration? Please place FarMenu.ini menus in two folders, launch User Menu (F2) from child folder and press BackSpace - will you get Main Menu?