5xx Server errors
505 HTTP Version Not Supported
The server doesn't support the HTTP version used.
Common causes
- A very old or misconfigured client or server.
How to fix it
- Use HTTP/1.1 or newer.
Find out the moment your site returns 505
Spot Downtime monitors count this as down and alert you, since visitors see an error. (The public “Is it down?” checker treats 4xx answers from big sites as online, because many refuse automated requests.)