This is an error produced by Salesforce not by Marketo so I recommend reaching out to your salesforce Admin to help troubleshoot the logic that is failing. A quick google search on "System.NullPointerException" returns some helpful information that should help you narrow it down.
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+1 on Kevin. Furthermore, it means there is an APEX trigger in SFDC that tries to read, update or insert info in another object, but the data for it comes up empty. Likely bad design of the APEX trigger itself.