A couple of questions
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Are your controls in a Repeating section, or all by themselves in the form?
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Have you tried connecting the secondary connection to a simple dropdown? Does it work?
Based on the error message, it seems as if it's a problem with the data source, not the control. Check out this article for what MAY be related - http://support.microsoft.com/kb/982247. They're saying something like you have a list that you're using, that has required fields, and the required fields are not filled in with data.
[EDIT] - I reread your message and you are using code to populate the control? I did a search on "Schema validation found non-data type errors" and found a lot of links, like the one above, that seem to indicate something about a "nil" field not being filled it.
http://sharenotes.wordpress.com/2008/03/23/infopath-forms-code-error-schema-validation-found-non-data-type-errors/