TL;DR
- Open "System Preferences > Privacy & Security", scroll down and click the pop-up menu next to "Allow accessories to connect", then select "Automatically when unlocked"
- Use the Yubico Authenticator app to configure the YubiKey with a PIN
Long answer
I noticed that when I plug in the YubiKey a window pops up for a millisecond and then disappears again. I made a screen recording and in the video was able to see that the popup asks for permission to grant access to this Mac to an accessory named "Yubico Yubikey OTP+FIDO+CCID".
Based on this, and on this Apple support article, I opened "System Preferences > Privacy & Security", scrolled down and clicked the pop-up menu next to "Allow accessories to connect", then selected "Automatically when unlocked" (instead of "Ask for new accessories" which was the default setting).
I still had the YubiKey plugged in and immediately a keyboard assistant window popped up, apparently to set up an external keyboard. I terminated this without going through the assistant.
After going back to the Yubico Authenticator, the app could now finally see the YubiKey, showing me the firmware version 5.7.1. However, creating a passkey with the website still failed.
The final solution was to set a PIN on the YubiKey using the Yubico Authenticator app. Once the PIN was configured, creating a passkey and logging in with it finally worked.
The remaining problem is that when I switch back the accessories setting to "Ask for new accessories", then unplug the YubiKey and plug it in again, the device is again not recognized. This prevents me from using the default, more secure setting "Ask for new accessories".
PS: For what it's worth, the problem described in this discussion sounds exactly like what I had. Unfortunately that discussion is already locked so I was not able to post my solution there. By linking that discussion here, maybe net search algorithms will pick up the connection.
