I'm not an Apple guy, so not sure, but I don't see any mention of a "Secret Squirrel" connection code in my AVM 70 documentation, which is as follows ...
"Depending on your network type, follow one of the procedures below to stream via AirPlay.
Wired Network Connection:
- To listen to audio via AirPlay on your processor, ensure that your Apple device is connected to the same network as the AVM.
- Select the processor as the AirPlay audio playback device.
Wireless Network Connection:
Ensure that your Apple device is connected to the same Wi-Fi network to which your AVM processor is connected.
- Enter the Apple device’s Wi-Fi Settings Menu.
- Locate the “AVM” device under the “Set up new AirPlay speaker” setting.
- Select the processor and allow it to connect.
NOTE: The processor is always listed as “AVM” in the AirPlay speaker menu.
I don't recall encountering any issue setting up Chromecast from my Android to my AVM 70, which is similar.