Hi,
Excuse me for reply in Emglish but my German is not there yet. Sometimes I have similar issues with the cabel/apple TV app while connected to the beamer. After trying different options (closing app, restarting beamer, reconnecying thr cable etc.), I noted that disconnecting cable, closing the app, restart of the iPad and connecting the cable again helps - the message about unauthorized display is not presented any more. Hope it helps in your case as well.
BR

What exactly did you want to look at? APP?
It's very difficult with the iPad (unless you do a jailbreak, but I wouldn't necessarily recommend that). These locks have existed for a long time on Netflix and Co.