I think your PHP pool is saturated:
- Apache idle during the downtime only serves static files
- idle timeout 160 sec + incomplete headers (0 bytes)
Apache forwarded the request to the FPM socket, but the PHP pool no longer has any workers available to handle the request, resulting in a timeout on the fcgi proxy side; FPM closes the connection without sending a header.
Unfortunately this is becoming more and more common; I see it every day on my hosts. htaccess actions have limits in the face of the democratization of this kind of practice (see an extreme case here Résistance aux bots - #18 par Sich). OVH or elsewhere it will be the same.
It will be hard to defend yourself against this cheaply while keeping your shared hosting. Cloudflare, with an investment to understand how it works, is certainly your best option.
@Sich [mode complot]I wonder if it's not CF that organizes such bot traffic to capture customers[/mode complot]