New issue with version 2.0.2:

Your transaction failed due to an error: [INVALID_CARD_DATA] Card nonce not found in this application environment. Please ensure an application ID belonging to the same environment is used when generating the nonce.

Have tried multiple cards.

Update: Aha! The issue is that if you are importing a live site database to the test site, you need to delete the Square Webpay app - it's not enough just to update the App ID and secret. Delete and re-add per the instructions and this issue will be resolved.