WordPress User Sync Issue

Hello,
The WordPress User Sync plugin is installed on both websites but the we are receiving this error message on the sub site, "There was a connection problem. Please check the URL and Key of the Master site."

The master site has an SSL (https) and we see another person had the same issue on the support forum earlier this year. Nothing is documented that there was a solution for the problem.

Please let us know if there is a solution.
Thanks!