Comment 6 for bug 1933027

Yep this got me today.

Unable to login due to YubiKey plugged in to the system. GDM sees the key and wants me to use it to authenticate, it's not set up for that and I had to pull the YubiKey to be able to log in with username & password.

Poking around in /etc/pam.d/ I found there's a few options ror smartcard configuration, and as a workaround just enabled one that allows both password and smartcard auth:

$ sudo update-alternatives --config gdm-smartcard
There are 3 choices for the alternative gdm-smartcard (providing /etc/pam.d/gdm-smartcard).

  Selection Path Priority Status
------------------------------------------------------------
  0 /etc/pam.d/gdm-smartcard-sssd-exclusive 50 auto mode
  1 /etc/pam.d/gdm-smartcard-pkcs11-exclusive 30 manual mode
  2 /etc/pam.d/gdm-smartcard-sssd-exclusive 50 manual mode
* 3 /etc/pam.d/gdm-smartcard-sssd-or-password 40 manual mode

Press <enter> to keep the current choice[*], or type selection number: