invalid json message

Hello,

I have an issue on my site.
I click on the send button on my form, the loader goes on spinning forever and I have this message in the console :

code: “invalid_json”
message: The response is not a valid JSON response

I tried deleting fields one after the other, thinking that maybe one of them was wrong.
Finally I only have one simple text field in the form and I still have the issue…

Could you please help me ?

Thanks
Regards