I believe I'm having the same problem, or at least something close. I've had paypal enrollments working just fine on Moodle 2.0.3+... However, over the last two days, I received the following message twice now:
Hello [my name],
Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:
If you do not recognize this URL, you may be using a service provider that is using IPN on your behalf. Please contact your service provider with the above information. If this problem continues, IPNs may be disabled for your account.
Thank you for your prompt attention to this issue.
Obviously, [my name] and [my domain] were set to sane things.
In my paypal account history, I received a payment TWO days ago that is flagged as an "eCheck payment" and is apparently "Pending Until Jul 5, 2011". Given the timing of it all, it would seem that this payment has trigged this issue in my case.
Further, paypal indicates that this payment is not yet complete and advises not to "ship" anything until it clears. However, the enrollment plugin has granted this student access to the course. (inferred via dates, names and enrollment method). Which makes me think students could possibly provide a bogus eCheck but still get enrolled.. Right?
Anyway, I'm fairly unsure what I ought to do about this. Perhaps I should look for a way to reject eChecks in my Paypal account?
I'd certainly appreciate some feedback!
UPDATE: I found that I could block eCheck payments in my paypal account profile preferences. So I've gone ahead and done that on the assumption that that's the issue here. Nice to also see that in changing that setting the pending eCheck payment is still pending and thus didn't seem to get clobbered. I'd still appreciate some feedback on this as I just acted on a guess.