No web_accept processor registered for this item_number; Status "subscribed" in Account screen, but the role is never assigned?

Issue Summary

I am trying to set up a simple pay per role subscription. My progress so far is that the user can click subscribe, accept, pay via PayPal, and return to My Account to see "Already subscribed". So that's working. But the user never receives the website role as designated in the lm_paypal setup screen.

IPN does reach me (albeit with a few warnings of ignored, unrecognized fields). I have enabled debug, but I am not sure where to go to find this debug output? Watchdog reports:
No web_accept processor registered for this item_number: 1

It's clear that 1 refers to my 1 subscription (and adding another shows "2"), but the function that produces this log doesn't seem to try to call anything but the donate function (which I haven't got enabled and don't need).

Am I doing something wrong? Maybe I missed something? Can I provide more info that will be helpful?