Appreciate the update @kevenespoir. Are you comfortable performing some cURL commands to help determine any IPv6 blocks? If so the following can be done from your web terminal or SSH:
curl -4 -I https://sitekit.withgoogle.com
The above will check the status if communication over IPv4
curl -6 -I https://sitekit.withgoogle.com
The above will check the status if communication over IPv6
Your hosting provider may be able to assist with the above. It’s possible that you’ll encounter a 200 response for the first command, and a 446 or other unsuccessful response with the second.
Alternatively, if you are able to create another WordPress site from the same hosting platform it might be worthwhile checking does the same occur. I ask as we’ve noticed some reports of blocks when sites are communicating over IPv6 only.
Let me know if you have any questions with the above.