What are the most common reasons a mobile user might see an

For a mobile user, the most common reasons for encountering Error 521 are primarily server-side or related to the website's hosting environment: 1) The origin web server is genuinely down or offline. 2) The web server's firewall is blocking Cloudflare's IP addresses. 3) The web server is overloaded, experiencing a crash, or has configuration issues. 4) Cloudflare's specific IP ranges are not whitelisted on the server. These issues prevent Cloudflare from establishing a connection to serve the content to your mobile device.