We recently had another seller encounter the same pair of errors, so I just wanted to post a followup here with the resolution that fixed it for them:
Paypal had a weird issue with the customer service number not being in the profile. It had not been there for the entire ten years that I have been a client. Then without warning the account could not process the credit cards without it. Paypal reps have no record of why this happened.
To clarify, that "customer service number" would be a phone number entered in the seller's profile at PayPal's end. This number being missing in the seller's profile was what caused the 10001 "internal error" at PayPal's end which prevented completion of checkout.
Apparently the 15008 error was a red herring, as that's just a "SuccessWithWarning" notification that happens fairly routinely without preventing checkout, and which our own system's total-verification routine compensates for. This error was only being displayed because it coincidentally happened to occur at the same time as the 10001 error which was the real show-stopper.