Hi Marco,
Given that Test was successful, the message sent from USS -> SDM looks for a matching screenname+email_address of the user in USS with userid+email_address of the user in ca_contact table of SDM.
For this specific case, you may want to look into <USS_Install_Dir>\OSOP\logs\Liferay***.logs for any error that USS obtained. Corresponding SDM logs stdlog.* and jsrvr.log should show some useful information about the attempted operation as well.
_R