I've imported my WSO2-DSS's certificate to my ESB keystore from configuration->KeyStores but it still does not work. It seems that it can not validate the host name! The error was this: "error: Host name verification failed for [MyHostName]"
I added following parameter to the <transportSender name="https
part of /repository/conf/axis2/axis2.xml file, restarted the server and it worked :)
<parameter name="HostnameVerifier">AllowAll</parameter>