I've retrieved the Elder Scroll in the Soul Cairn, but I can't leave because the purple barrier which I guess isn't supposed to be there at this point is preventing me from getting back to the portal. What do I do?

The "purple haze" (barrier) only disappears in front of . You still have to navigate around it to get back to the stairs out. There is a path through the wall near that guy with the cart selling things for soul husks.

Thanks, found it.