Jenkins is up-to-date, although all plugins.

If I go to "Manage Jenkins" and want to configure nodes for master/slave configuration via "Manage Nodes", I only get this single Plain-HTML-response in the web-browser (without the asterisks)

*****************************************************
Jenkins-Agent-Protocols: JNLP4-connect, Ping
Jenkins-Version: 2.121.1
Jenkins-Session: 0cd17b98
Client: 172.17.0.1
Server: 172.17.0.2
Remoting-Minimum-Version: 2.60
*****************************************************

No GUI, nor anything else, only this text in the HTML-Body-Tag.