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Solved: 'Please check connection, Mattermost unreachable' After Deploying New Mattermost Version

Solution from here:​

[Solved] Error: Please check connection, Mattermost unreachable. If issue persists, ask administrator to check WebSocket port

The specific fix is to modify your nginx configuration:

    location / {
        proxy_pass https://127.0.0.1:8443;
        proxy_set_header Host $http_host;
        proxy_set_header CLIENT_HOST $remote_addr;
        proxy_set_header Origin "";
        # Add the following section
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_ssl_verify              off;
    }

The reason is: nginx reverse proxy cannot handle websocket connections. The error message contains: error=connect: upgrade websocket connection failed, websocket: client did not use websocket protocol