That would indicate your Common Notification URL or Payment Variable Information URL is not responding. If you are using that latter setting for PDF Stamping, then there is something amiss with your file, so the Stamper cannot open the file to modify it. As I recall, we helped you get this working recently, so have you uploaded a new PDF file to replace the one that was working earlier? If so, examine the settings for your new file to make sure it meets our Stamper's requirements.
If you get the Stamper working with a given file, it should always work reliably with THAT file. If you upload a new file, you need to retest to confirm the Stamping will work with the new file and, if not, figure out what settings are different in the new file that's made it incompatible with stamping. The usual problem regards password security settings, either using an incorrect author password in the Stamper URL, or saving the file with a security compatibility level set too high.
We recommend uploading PDFs without any original password protection, because the Stamper can generally handle those flawlessly, and the Stamper always adds its own, randomized author password to each file anyway. A read-only password where you give out the password along with the file is about as secure as putting a lock on something but leaving the key in the lock; anyone who wants to share that file will share the password along with it, so what's the point? :^)
