My Safari using clients were having issues with this error message when trying to use the Editor:
“We’re having trouble with your browser’s cookies. Make sure your browser accepts third-party cookies.”
In Safari, however, even with the box to “Block All Cookies” unchecked, clients still get this message. The only forum posts I could find list solutions that relate to older versions of safari. The solution I found is to also uncheck the box in the Privacy Settings to “prevent cross-site tracking”
Then refresh the page.
Once unchecked this error message went away.
I would have posted to a relevant forum, but they were all locked so maybe an admin can place this solution in an appropriate place.
I hope this helps someone.