We had a similiar situation...

Do you have both Reader and Writer installed on the machine you're accessing from where the reader was installed first and writer was installed after the fact?

If so, try accessing the page that delivers the pdf thru a machine that only has reader or writer installed.

We can't figure why this causes a problem, either - but if you access from a machine that only has one or the other installed it'll work just fine.

I know this won't fix you're problem but it'll at least tell us if there is a problem with both reader and writer being installed...