Please send the plug-in log file to support@splendidcrm.com. You will find it at:
C:\Documents and Settings\[user]\Application Data\SplendidCRM\SplendidCRM Plug-in for Outlook Log.txt
What version of the SplendidCRM application are you running? The SessionID problem can be isolated to the soap.asmx file, so having an old version can be the problem.