I made a new API workflow and when setting it up used ‘Detect request data’ and used the /initialize url to test the webhook. Then I remove /initialize from the url so that stripe sends the normal requests to the right url. When I send a test request then I get this message:
I had the same error. In my case, the problem was that the Webhook trigerred a “Send email” action in the workflow. In this action, the Subject field was empty. Everything worked once i added the subject.
Wow, what a shocker that one could address an error message that says “HEY YOU NEED TO INCLUDE A SUBJECT LINE!” by adding a freakin’ subject line. Floored.