Why unlocking the user session locked by screensaver, using smartcard fails?

Issue

• After screensaver locks the screen, or user session being locked manually, it prompts for smartcard PIN to unlock the screen. Entering the PIN, system always responds with: "Authentication Failed".
• PIN is correct as CaC does not get locked. To confirm, click on "Switch User", which prompts for "Smartcard Authentication". Entering the same PIN, screen unlocks.
• Debugging enabled for pam_pkcs11 and respective mapper, generates below log, when unlocking screen fails:

 Jan 25 08:14:13  gnome-screensaver-dialog: CN mapper started. debug: 1, mapfile: file:///etc/pam_pkcs11/cn_map, icase: 1
Jan 25 08:14:13  gnome-screensaver-dialog: UniqueID mapper started. debug: 1, mapfile: none, icase: 0
Jan 25 08:14:13  gnome-screensaver-dialog: pwent mapper started
Jan 25 08:14:13  gnome-screensaver-dialog: Null mapper match set to 'never'
Jan 25 08:14:13  gnome-screensaver-dialog: trying to map & match CN entry '<cn-name>'
Jan 25 08:14:13  gnome-screensaver-dialog: Using mapping file: 'file:///etc/pam_pkcs11/cn_map' to search '<cn-name>'
Jan 25 08:14:13  gnome-screensaver-dialog: parsing uri:
Jan 25 08:14:13  gnome-screensaver-dialog: protocol = [file]
Jan 25 08:14:13  gnome-screensaver-dialog: user = [(null)]
Jan 25 08:14:13  gnome-screensaver-dialog: password = [(null)]
Jan 25 08:14:13  gnome-screensaver-dialog: host = []
Jan 25 08:14:13  gnome-screensaver-dialog: port = [(null)]
Jan 25 08:14:13  gnome-screensaver-dialog: path = [/etc/pam_pkcs11/cn_map]
Jan 25 08:14:13  gnome-screensaver-dialog: opening...
Jan 25 08:14:13  gnome-screensaver-dialog: get_from_uri() error: get_file() failed: open() failed: Permission denied
Jan 25 08:14:13  gnome-screensaver-dialog: Error processing mapfile file:///etc/pam_pkcs11/cn_map
Jan 25 08:14:13  gnome-screensaver-dialog: get_unique_id() failed
Jan 25 08:14:13  gnome-screensaver-dialog: Trying to match pw_entry for cn '<cn-name>'
Jan 25 08:14:13  gnome-screensaver-dialog: CN '<cn-name>' doesn't match login '<user-name>'
Jan 25 08:14:13  gnome-screensaver-dialog: Provided user doesn't match to any found Common Name