When I entered my reservation number into the Hertz website I got the message "View a Reservation - Please verify the confirmation number and try again. [NRX136]". Hertz phone support told me that if you book your car through a third party service, Hertz doesn't recognize the confirmation number. Which I don't think is true because the confirmation number came from a Hertz email address, not from Priceline. It's the letter K followed by 10 numerical digits.

What I think happened is that I started to confirm the reservation the previous day and was interrupted. Then when I came back to resume confirming the booking, I couldn't confirm it anymore. I think there's a bug in the system that doesn't handle a partially confirmed booking properly. Hertz continued to nag me to confirm the booking (via email and SMS) without providing me a way to do so.