I got a weird issue. Tried to connect using
websocat ws://localhost:9090/ws
and got this error:

websocat: WebSocketError: I/O failure
websocat: error running

if I change the hostname to the IP address it works well.

my /etc/hosts has localhost records

127.0.0.1 localhost
::1 localhost

telnet command works well with the localhost hostname

$ telnet localhost 9090
Trying ::1...
Connection failed: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.

I guess it tried the IPv6 address, got connection refused, and exited.