Skin not allowed in multiplayer

My world has become unplayable since updating to 1.12. Each time the program crashes I am informed that I am using a skin from a content pack that is not allowed in multiplayer. I am using either Steve or Alex. I have disabled multiplayer but still the problem continues. I have also created new worlds which do not crash as much as my established world which is 575 MB. but none the less are still unstable.