After over 40 minutes the rotating blue circle continued to rotate, so I closed the Paypal browser tab. I then navigated to the ACP/PBP site and see "To be paid !" next to my entry number, followed by Modify, Documents, Pay, Cancel buttons. Pressing the Modify button takes me to my registration details page which shows the details I entered before failing to pay.
I might make another attempt to pay later today. I don't really want to have to create a Paypal account just for this; I've avoided that scourge so far.