Apple TV 3rd gen (1469) - cannot connect to WiFi - password issue

Hi, I cannot connect my 2 old Apple TV 3rd gen to the WiFi any longer.

It says:

• • "There was a problem connecting to the network.
  • The Wi-Fi password you entered is not correct. Try again. (-100)"

1. there is no option to type in a correct password or to forget the network connection.
2. I tried to choose "other" and typed the WiFi SSID manually. It jumps to the same error message.
3. I renamed my WiFi network's SSID and selected it from the list. It jumps to the same error message! (my logic is leaving me here!)
4. I reset to factory settings via the TV. It didn't change a thing!
5. I reset to factory settings using a USB cable via iTunes on my Mac. Same effect.
6. Connecting via ethernet is no problem as all.

I am using an EERO-Pro mesh network.

Is this a subtle hint from Apple to bin my 2 old Apple TVs?

Anyone having an idea?