PSA: If HP warranty dispute form doesn't work, read this.
If you are trying to submit proof of purchase for warranty validation you may have encountered an error like the one pictured above. I had the same experience with HP's website (see my post here). I was given the runaround at first and the attention received from the HP "Social Media Ambassador" was entirely unhelpful. I hope this information helps someone else here.
This is what finally worked for me (to get the form submitted successfully):
Use the "Dispute" link at the bottom of the warranty results page.
Fill in all required information on the dispute form.
Click the radio button indicating you'll attach proof of purchase (even if it's already selected).
This is the important part: attach your receipt in a format OTHER THAN .PDF (I ended up using a .PNG image file).
The website indicates that a .PDF file is acceptable but the times I tried submitting the form with a .PDF attached the website returned the error pictured above. After submitting with .PNG image I promptly received an automated email with case number. Within 1 day I'd been notified that proof of purchase had been received and the warranty status verified and updated.