Error "There is a problem loading this content"

I bought a new unused Apple TV Gen 3.

That’s kind of impressive in its own way, as those have been discontinued for more than 5 years. That model could now be considered “vintage” and officially unsupported, although not officially listed there yet.

I connect it with my iPad Air2. Everytime I try to stream any content from my iPad to Apple TV I get this error "There is a problem loading this content".

What are you trying to stream from the iPad? Because that matters for compatibility.

• a file on the iPad (← may not work depending on codecs)
• an online stream from the iPad through a web browser (← may not work depending on website)
• an online stream from the iPad through a different app (← may not work depending on app/service)
• a file accessed from the iPad, when that file is located on another home networked device (← won’t work)

Use AirPlay to stream video or mirror the screen of your iPhone, iPad, or iPod touch - Apple Support

If screen mirroring or streaming isn't working on your AirPlay-compatible device - Apple Support

The video format might not be compatible with the ATV. Not all mp4 videos are the same. I have issues with some of my older recordings and have to process them with a video converter app before they will play on my ATV. Mostly it seems related to the audio component.

I have ATV 2.

Apple TV+ requires an Apple TV (3rd generation or later), even for AirPlay.

All the ways to watch Apple TV+ - Apple Support

Same issue here. I have ATV 2. But using iPhone 12 and latest iPad to airplay Apple TV+. Never works. If Apple put Apple TV+ on the ATV, it would take care of my needs but no luck.