Since you can see the “FR_Télécharger_CahierDesCharges” form and the associated submissions in the HubSpot UI, theoretically it should still work.
According to the documentation:
Note: This endpoint can only be used to get submissions for the following form types:
- HUBSPOT - HubSpot marketing forms
- FLOW - HubSpot pop-up forms (formally lead flows)
- CAPTURED - Data collected for non-HubSpot forms
I am guessing your form would be classified as “CAPTURED” and should still qualify.
Have you tried to use the API endpoint with a different form just to make sure you have the API call set up correctly or that it works in your portal in general?