Is your site using an address starting with "https:// (aka SSL)?
If so, then you will need to remove its security setting. I'm not familiar with SquareSpace, so I don't know where it its options it might sit.
My post on
GregHelp provides a little more information on Nabble and SSL
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.