This was working well and all of a sudden it isn't, so I'm not sure what happened.
We have one level of membership and it is free. (well, two if you count the guest level).
You go to the registration page, click the Subscribe button, and it launches the pop-up registration form. It is configured so the customer does not have to choose a subscription:
After my client reported that both she and her customer saw the error, I tested it and I also get the error. I checked the backend, and the email I used to register in this test is not listed as a member. I am sure I'm filling the form in completely and correctly.
What could be the cause? As I say, this form was working a couple days ago. The only thing I changed in the code since the last time it was used successfully... was to change the notification email that goes to the admin from the appointments+ plugin.
I haven't touched the membership code.