I did fix it, but don't have the slightest idea how or why it is now working. In fact everytime I create a new one, I spend about a day messing around with it trying to get the new one working. Some of the things to review:
1. Make sure your form xml matches exactly. What I did was create the form, name my data fields and then save it. I then open the PDF in Acrobat, populate every field as a user would then export the form data as XML. I then go back to use this XML as a sample data source as well as using it in my PHP code to ensure it matches.
2. I had to use a two step process to read the file into the browser. pdf_formname.php (sets the correct header and fopens/echos the content of the next file). xdp_formname.php (reads from the database and exports the correct xml). Very likely this is not at all needed, but since it is working, I'm afraid to touch it.
3. Make sure your html reference at the bottom of your XML/XDP is pointing to a real file.
4. Cleared my cache more times that I can count.
Sorry I can't be more help. I just wish there was a better debugging tool from Adobe that would tell me what file it thinks is missing rather than a generic message.