[andrewas]

You cannot use a ring, amulaet, cloak or other item of protetection, while wearing any armour with a bonus to armour class. _any_ armour. Including Red Dragon Scale - although it was orignaly bugged so you could do this.

You also cannot wear any such item in combination with another.

The only two armours i am not sure about are Drow Plate and the BMU, which is secret and might or might not allow this.