Note to Luke McDonald:
my client and I tried various solutions, contacting Stripe tech support and hiring Naomi to help us with troubleshooting. When she put the Stripe AddOn into test mode, the problem went away, even when she returned it to Live Mode. But then it resurfaced a few days later (with a somewhat different symptom: instead of the error message, a continuously spinning cursor – but the money still went into the account).
What seemed the fix the problem for us (or at least, it hasn’t resurfaced for 10 days now) was that we went to our Stripe account dashboard and updated the API from Nov 7th version to the Feb 13th version.)
Naomi said it shouldn’t have made a difference, but (apparently) it “jiggled” something and it did.
