Problem connecting to mosquitto MQTT Server with authentication

I have a problem connecting to mosquitto MQTT server using authentication (without TLS and without websockets). I use the client the following way (I modified the Client demo):

MQTT.Client := WSClient;

txtParameters.Text := '/';
chkTLS.Checked := False;
MQTT.Authentication.Enabled := False;
MQTT.Authentication.Username := 'openhabian';
MQTT.Authentication.Password := 'mypassword';
MQTT.MQTTVersion := mqtt311;
txtHost.Text := '192.168.211.51';
txtPort.Text := '1883';
chkTLS.Checked := False;
chkOverWebSocket.Checked := False;

When I connect the Client displays immediately:

#connected
#disconnected
#exception: Connection Closed Gracefully.

The mosquitto log displays only:

1580681116: New connection from 192.168.211.99 on port 1883.

What am I doing wrong? Connections to this IP using MQTT-Explorer using the same connection settings work correctly. The "Client" connects correctly to test.mosquitto.org but without authentication but not to my mosquitto server on raspbian.