I have a solution! I spent the afternoon on this case with Mike_H and it appears that it's related to the way Blackpearl generates the data connection string in Infopath manifest.xsf file.

Just give a try:

• from the form library where you published the form, navigate to advanced settings, and right-click "Edit Template" and download it (Save Target As).
• open it in Design mode
• in file menu, select Save As Source Files
• edit the manifest.xsf with a text editor and find where the data connections are set

You should see this kind of tag:

<xsf2:webServiceAdapterExtension ref="Get Workflow Task Actions Service" trackDataSetChanges="no">
      <xsf2:connectoid connectionLinkType="relative" siteCollection="http://myserver:10000/sites/mysite" source="/K2 BlackPearl Data Connections/InfoPath Web Service/Get Actions.udcx" name="Get Actions"></xsf2:connectoid>
     </xsf2:webServiceAdapterExtension>

Just remove the initial slash ('/') in the "source" parameter, and add a final slash ('/') at the end of the siteCollection parameter, so that you finally get this kind of data connection:

<xsf2:webServiceAdapterExtension ref="Get Workflow Task Actions Service" trackDataSetChanges="no">
      <xsf2:connectoid connectionLinkType="relative" siteCollection="http://myserver:10000/sites/mysite/" source="K2 BlackPearl Data Connections/InfoPath Web Service/Get Actions.udcx" name="Get Actions"></xsf2:connectoid>
     </xsf2:webServiceAdapterExtension>

 Then open your manifest.xsf in design mode, save it as an XSN file (you can overwrite the one you downloaded from the form library), and publish it in the same library. Open it, it works!

Hope this helps!