Thanks for that!
I checked my "Cookies to support" config and I had:
which I changed to :
but I still got error messages. These cookie are associated with the user preferences for the layout of the YooTheme templates.
I checked my HTTP cookie headers in the browser and I couldn't find these cookies. I deleted the entry entirely and the messages have gone. Not sure what the impact is, but the error messages have now gone. Good enough for me ;-)