Error: Heartbeat detected no valid endpoint.

[seehat]

After 5 minutes of having a connection to my /ui Dashboard, I get the following error

in the nodered-logs:

Heartbeat detected no valid endpoint. Please check your connection settings (Base URL, Serving Port and Protocol).

and in the dashboard with a yellow dot:

aktiv - kein Webserver erkannt

Can this be an error in the code? - I can't figure out what to change in the settings. The path /ui is accessible locally via my browser.

Config-Node-Settings:
Host: localhost
Port: 1880
Protocol:: http
Url: /ui