There was an error in our site CSS that we fixed (thanks, Saga from Squarespace!) and then Squarespace had to clear a cache for the error to disappear in our case. I also had to clear my browser cache again for the fix to show up on my end.
I still don't think that was causing the error itself though, as you and others who have cleared all custom CSS (and those who have never had custom CSS) are experiencing this.
You'll need to contact customer support and ask for assistance and make sure to ask them to clear the site cache, then clear your browser cache as well.
