As already reported by some users. Factory reset.
1. while the Chromecast is plugged into the TV and switched on, press and hold the button on the back of the Chromecast. The LED should start flashing amber.
2. when the LED light is solid white, release the button and the Chromecast should then reset.
During setup, enter your Google login details and select everything you need.

Alternatively, you can also "project" YouTube etc. directly onto the screen via Chromecast using your mobile.

Hello, I have 3 of these devices in use. And they work perfectly for me. Of course, you need a login for all the applications listed. Why don't you download the Arte. ZDF and ARD app and see if it works. They are sure to work, even without a login. If they don't work, it's because of the box or your Wi-Fi connection. But if these 3 apps are running, it's because of your logins. Then you have to go over the books. Incidentally, I'm more than enthusiastic about the Google Chromecasts, which is why I've bought one after the other to replace outdated Apple TV boxes. The price/performance ratio is simply convincing.

What error messages are you getting?

As beni1971 says, definitely try a factory reset!

Hi, this is difficult to understand.
Have you tried a factory reset and then using a different google account?