WordPress User Syncronization is not syncronizing my users


I just installed your plugin in two different domains: http://www.trasmittanza.org and http://www.dartwin.it
I set the first as master and the second as sub.
Following exactly your instruction I started the syncronization but I constantly receive an error message "There was a connection problem. Please check the URL and Key of the Master site"
Url and key are exactly those shown in the master site (I used copy/paste).
The subsite it is shown as registered and connected in the master site, while there is no sign of succesful activity in the subsite.

Please help me in solving this matter.

Thank you