Importing MPP file and creating XMLFile with InputStream fails

I'm trying to upload an MPP file to my asp.net page then create either an MPPFile, MPXFile of XMLFile from the InputStream of the uploaded file. It simply fails each time with an 'Invalid file format' exception.

Has anyone experience similar behaviour?

Is there something blindingly obvious I have missed?

Thanks

I’m affraid we didn’t have such exceptions in the last 2 years.
What about inner exception? Could you send this file to me for testing?