You have some kind of security protection or redirection feature enabled which is not allowing the HTTP Challenge to Apache to work. When the Let's Encrypt server sends this challenge it should receive the verification token. Instead, your server is sending it some sort of OVH redirection page. Is there some feature to automatically redirect all HTTP requests to HTTPS?
I don't know OVH well enough to instruct how to disable that. But, review your settings. Another option is to use the DNS Challenge but this is often more difficult to setup
Here is more output from a sample HTTP Challenge I made from my own test server:
Note the server is "nginx" so not Apache. This is clue that OVH is intercepting this request or maybe some other network service.
Also note the IP address in the href belongs to Orange and is not related to the IP of my server