NextcloudPi Fido2 not possible to add webauthn device

Hi,
I´m using NextclouPi (NC 20.03). I would like to use my Nitrokey for passwordless login. When I klick on “add web-authn device” the authorisation popups only for a moment. So it´s not possible to the FIDO2 stick.
Is this a problem of NextcloudPi? I´m additional using another installation of a “normal” Nextcloud. There it´s no problem tu use FIDO2.
Creating a complete new clean NextclouPi (!) installation didn´t help.
Can you help me?
Thanks in advance!
All the best