BatchConvert is used in the ActiveX interface of CDintf and BatchConvertEx is used in the DLL interface.
They are both mentioned in the Developers' manual, you will find information about BatchConvert on page 145.
After rechecking your code I don't think it is an issue whether you use BatchConvert or BatchConverEx.
I tried to use BatchConert from VB and I didn't get any error message. Please send your code to firstname.lastname@example.org
so we can try to reproduce this issue from C++.
I guess you may be using an older version of CDintf with version 2.10e of the PDF Converter, please check that you only one version of CDintf210.dll on your sytem and that it is the latest one.
Hope this helps.