HexChat, at some point I don't remember, started disconnecting periodically. It didn't interfere with my use of IRC, but it really cluttered up my chat windows, because I'd also get the usual channel connect messages.
Disconnected (No such device or address)
The solution seemed to relate to a setting that was introduced in a HexChat version, but wasn't set in existing configurations.
/set net_ping_timeout 120
You can read more in comments by TingPing on GitHub where I got this solution