Most of the issues related to screendesigner are connection issues as sometimes clients have many layers of servers or vpn's or firewalls or certificate issue which may hinder connection and SD gives a common error saying username and password are incorrect. SD allows java debugging and we can enable ssl debugging and check at which server it stopping it from connecting.
Please follow the below steps to enable ssl debugging:
-> Open eclipse.ini file and the below java arguments
--Djavax.net.debug=all ( all/ssl can also be used)
->Save and open cmd at this location
-> run the command "eclipse.exe -debug"
This should open a java debug window as well as application, put in the url and check the java debug window