Form stuck back from Stripe payment | NF Support can’t resolve

Ninja Forms 3.4.25
Stripe 3.1.3
Wordpress 5.5
Theme Twenty Seventeen 2.4
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Hi everyone,

I am now blocked since one week on all my forms with the same problem.
Back from the Stripe payment page (by canceling or executing payment) :
– my form stays stuck with only the submit button displayed, with the processing label on it.
– no any user or administrator mail is triggered
– the submission is stored whether I execute payment or cancel it !!
– the payment, if executed, appears without any problem on my Stripe dashboard. Hence the problem occurs back to my Ninja form.

In order to isolate the problem, I have :
– deactivated all other plugins than Ninja Form add-ons
– tried 2016 and 2020 default WordPress themes
– deleted and re-installed Ninja Forms main plugin
– downgraded to WordPress 5.4

I still have the issue. Impossible to resolve it. It probably came with one of the latest Ninja Form updates.

I am in contact with Ninja Form support since one week, I gave them administrator credentials, and yet, they are unable to find a solution.

So I write here hoping someone else had a similar issue and could help me with it.

To reproduce the issue :
1. Go to https://www.salsa-brest.fr/essai-form-2
2. Fill in the form and click on submit button (Envoyer). You’ll arrive to Stripe payment page on test mode.
3. You may try to come back by canceling payment or execute it with a tester Stripe card.
4. Observe that back to my site, the form stays stuck with only submit button displayed on it.

Please let me know if you have any idea where this issue may come from.

Good day

The page I need help with: [log in to see the link]