File does not begin with %pdf-

We are using fpdf to dynamically create a pdf based on search strings against a database.  On our test box it works just fine, but our production boxes return "File does not begin with %pdf-".

All of the boxes are Windows 2003 SP2 running IIS6.

I've gone through the config for IIS6 on the test box and compared it to the production boxes.  (We load balance the site, hence the two production boxes.)  The MIME file type for .pdf is set to application/pdf.  I've tried application/x-pdf as well and gotten the same result.  

I saw a mention of KB944533 potentially interfering, but that is not installed on any of the servers in question.  .NET version is set to the same (1.1) on all of the boxes.  I've tried 2.0 and same result.

As I said, the .pdf displays on the client when browsing to the test box, not from the production boxes though.  To make things better, if I try to browse to a .pdf directly on the production boxes, it works fine.

Odd issue, so any insight is welcome.