Mixed content on submission

Hi,

I have an error when submitting a form :

Mixed Content: The page at 'https://www.mywebsite.com/fr/' was loaded over HTTPS, but requested an insecure resource 'https://www.mywebsite.com/fr/wp-json/contact-form-7/v1/contact-forms/543/refill/'. This request has been blocked; the content must be served over HTTPS.

index.js?ver=5.5.6.1:1 
{code: 'fetch_error', message: 'You are probably offline.'}
code: "fetch_error"
message: "You are probably offline."
Prototype: Object

The result is that the success / error message is never displayed.

Can you please tell us how to solve that ?