Tried everything, including cleaning the front roller ball. Nothing worked.[br]
However, I have two Roombas (different models), one upstairs on carpet and one downstairs on wood floors. I swapped them. The one that used to get the error works perfectly on the wood floors. The other one is doing a great job on the carpet. So there was something about the thick pile that for some reason started giving my other Roomba fits. Go figure.