PayPal Pro duplicate payment error

I've been using Pro Sites for a while with PayPal Express, but I finally signed up for PayPal Pro. When I enable it and try to upgrade a site, I get the following error:

Whoops, looks like you may have tried to submit your payment twice. You may have already paid! Check your subscripton info below to confirm

(Note the typo: subscription is misspelled.)

Nothing shows in the Account History table in the Pro Sites admin, and nothing is showing in my PayPal history either.

I had previously set that account to permanent status, and revoked it specifically for testing. I saw one other person mention this error, but they never posted a resolution.