Network timeout or antivirus blocking the connection. Solution: Disable HTTPS scanning temporarily in your antivirus, or use a VPN to change regions. Alternatively, download via torrent (if offered) or a static mirror link from the official Discord support channel.
GitHub - mioclient/oyvey-ported: super kosher minecraft client base · GitHub. Mio Client Download
For organizations deploying Mio Client across multiple workstations: Network timeout or antivirus blocking the connection