Hi @sorg! I also could not get U2F working with my Yubikey. However, I was able to make Yubico OTP work which requires setting these two environment variables:
YUBICO_CLIENT_ID
YUBICO_SECRET_KEY
These are obtained by signing up for YubiCloud at this URL: Yubico API key signup
After setup, I can even use the YubiKey 5 NFC and hold it to the back of my iPhone to provide a secure hardware second factor to BitWarden on the iPhone. It’s pretty sweet and I wish that 1Password supported it, but they have been reticent almost to the point of obstinate about not doing it for some reason.