Since upgrade from Leap 15.5 to 15.6 I'm no longer able to login with GDM when
my Yubikey is inserted.
The system is configured to log in using pam_u2f.so requiring me to touch the
Yubikey device to log in.

However since newest version, I get a username-field and a non functional
'back' button. Whenever I enter a username, it just refreshes and requests the
username again. No password is asked, neither is asked to touch the Yubikey.

In the logging I find: 
---
gdm-smartcard][2964]: PAM unable to dlopen(/lib64/security/pam_pkcs11.so):
/lib64/security/pam_pkcs11.so: kan gedeeld objectbestand niet openen: Bestand
of map bestaat niet
gdm-smartcard][2964]: PAM adding faulty module: /lib64/security/pam_pkcs11.so
---

So I installed pam_pkcs11
Now when I enter the username in GDM it tells me to start smartcard
authentication, and the light on the Yubikey lights up. When I touch it, it
flickers once and then stays on again. Never proceeding login.
In the logging I now find:
---
gdm-smartcard][31888]: argument card_only is not supported by this module
gdm-smartcard][31888]: Failed to initialize crypto
gdm-smartcard][31888]: pam_pkcs11(gdm-smartcard:auth): Failed to initialize
crypto
---

I also found this upstream bug: https://gitlab.gnome.org/GNOME/gdm/-/issues/877

It seems the yubikey is now detected as a smartcard despite it not been
configured as a smartcard.
I also don't find understandable documentation describing how to configure Pam
/ GDM to accept the certificate on the Yubikey for a specific user.
Currently I can no longer log in using my Yubikey