it has happened to me as well on XR1000 i believe theirs a ways to fix it and 1 way on whats causing it
firstly whats causing is it seems that when you do enable priority feature and login into web Interface it causes conflictions for whatever reason
secondly how to fix it is by having it still enabled via the app but then manually importing a rule after that head over to interface and error should be gone
thirdly what kinda sucks (for now) is if we disable the default Priority Rule and we add a manual rule Traffic priority will not actually work without having default rule enabled 1st and then have your manual rule added