Basically the same code as the original poster:
sPath="C:\\base.xml"
Dim ipApp
Set ipApp = CreateObject("InfoPath.Application")
Call ipApp.XDocuments.Open(sPath,8)
... some more code ...
But, if I replace the Open-call like this:
Call ipApp.XDocuments.Open("c:\\base.xml",8)
then the script works fine.