The Forms Server seems to be confused with your xpath... to check if this is the case, try to see if you can re-write it without using the prefixes (start with s1 and keep going up). You can rewrite this without s1 as this:
/dfs:myFields/dfs:dataFields/tns:SubmitForm/*[local-name()='newForm']
I think what is happening is that you have a secondary data source with an open schema which does not define what goes under tns:SubmitForm (or maybe even dfs:DataFields). The server cannot analyze the schema and thus gives you the error. The regular client does not analyze the schema so it's not an issue.
See if my suggestion fixes your problem.
David Airapetyan / Software Services ArchitectQdabra® Software / Streamline data gathering to turn process into knowledge