Maybe Ancestry wants you to be logged in from one device only? That seems at variance with what I've read from RootsMagic people and heard from other users.
I was never connect to Ancestry.com through RootsMagic (or though anything else) by more than one device at a time when this occured. RootsMagic does explicitly mention using multiple devises, and I don't recall hearing that you shouldn't be logged onto more than one at a time. Nothing ever works perfectly the first time out. So I have made known my experience! It is a functionality greatly to be desired!