Problem submitting form with Stripe

Hello.
I’ve a form which invites visitors to pay to become a member. When I submit the form I get the following console error in Firefox:

Uncaught (in promise) TypeError: undefined has no properties

I’ve switched off all caching. The form’s method is “page reload” and “load form using ajax” is switched off. Stripe is currently in “test” mode.

Can you help please?

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