can't synchronization sites and shows There was a connection problem.

I installed plugin on both site. then do what instructions told me .
when I click "Connect this site to the Master site, and do a FULL synchronization"
,it shows
There was a connection problem. Please check the URL and Key of the Master site.
I have double checked the master site url and the key.
Also I activated the Debug Mode.
When I use FTP to see if there is debug files , I only see there is only one in sub site.
I open that error_s.log , and see
[2014-10-24 08:19:56][S] 01 - new subsite conection - central_url=;; replace_user=0;; overwrite_user=0;;***
[2014-10-24 08:19:56][S] 02 - sending request - url=;;***
[2014-10-24 08:19:56][S] 03 - sending request - response=

I see no response in 03.
<p>The server can not find the requested page:</p>
in the bottom of that file .

Please help me with this.