Building with SSR false starts (and doesn't close) listeners

In my app I create a WebsocketServer on Port 3001.
If I run

npm run build

npm run build

with default SSR setting (

ssr: true

ssr: true

) everything works like a charm.

However, turning off

ssr

ssr

, the WebsocketServer gets started and not closed => EADDRINUSE error. Cannot run

dev

dev

start

start

afterwards.

After forcefully terminating the process, I can run

start

start

with the built files and it works.

Any suggestions on how to avoid this?