When Confirm is disabled on the sideboard screen, I would show the reason beside the counter. The red count is a useful warning, but a short message could explain which card move needs fixing
Good call, done for 1.0.44! On Android, when the sideboard count turns red and OK is disabled, a short line under it now tells you what to fix: “Move 1 to the sideboard” or “Move 2 to the deck”. On PC the dialog already won’t let you go over the limit, and the button tells you how many more to pick.