<p>➀ The error indicates a failure in establishing or maintaining a connection with an upstream server before HTTP headers were processed, leading to an abrupt termination; </p><p>➁ Potential causes include server overload, network instability, misconfigured proxies, SSL/TLS handshake failures, or security policies blocking the connection; </p><p>➂ Troubleshooting involves verifying server availability, inspecting network configurations, reviewing security/SSL settings, and analyzing server/client logs to identify the root cause.</p>