For some reason Rambox didn’t have an icon on the tasklist and it looked following way:

rambox-no-icon

Adding following lines to the signal handler finally resolved the issue (note that default configuration already has this handlerg):

client.connect_signal("manage", function (c)
    if c.class == "Rambox" then
        local icon = gears.surface("/home/pmakhov/Pictures/256x256.png")
        c.icon = icon._native
        icon:finish()
    end
end)

rambox-with-icon

Based on this SO answer: stackoverflow.com/a/30379815/1252056