There was a connection problem. Please check the URL and Key of the Master site.

Plugin installed on the 2 sites I want to sync. Correctly set up one as master and other as sub. Enter URL and key, making sure not to have extra spaces before or after in the copy/paste. Get error:

There was a connection problem. Please check the URL and Key of the Master site.

Tried this several times to no avail. Enabled debug mode - here's the log:

[2014-06-05 21:04:29][S] 01 - new subsite conection - central_url=http://digitalmediacube.com;; replace_user=1;; overwrite_user=0;;***
[2014-06-05 21:04:29][S] 02 - sending request - url=http://digitalmediacube.com/wp-admin/admin-ajax.php?action=user_sync_api;;***
[2014-06-05 21:04:31][S] 03 - sending request - response=0;;***

Ideas?