@phaedrux Thank your for the information. Also thank you Dougal1957 for coming up with the M575 command. I missed the part that I need it in the newer firmware. With this command the PanelDue with its version 1.17 connects to the DuetWifi with firmware 3.3.
Unfortunately I get a lot of guests tomorrow which stay until the end of the week. So I am busy preparing different things. I am going to open the case of my PanelDue and connect to it via USB as infiniteloop suggested on Monday in order to find out which version I have. I will let you know as soon as I have more information about it. And even though we have just Wednesday now, I wish you all a nice weekend!