I've had problems with this too. It's been going on for months. It thought it was because I was on the Beta for Ulysses, but it should be working now, right?
I've attached a screenshot of what I get as error message. Can't find any discussion of this. Help would be much appreciated, because I really rely on Ulysses for quick notes, and will do at least until Drafts 5 comes to the Mac…