As reported downstream:
https://bugs.gentoo.org/show_bug.cgi?id=477954

From time to time, I am unable to unlock screen from gnome-shell locker, I see a yellow "Authentication error" message below password box, messages like the following are recorded in journalctl:
Jul 23 16:53:24 siegeengine systemd-logind[541]: Removed session c1.
Jul 23 16:53:24 siegeengine systemd[1]: Stopped User Manager for 107.
Jul 23 16:53:24 siegeengine systemd[1]: Stopping user-107.slice.
Jul 23 16:53:24 siegeengine systemd[1]: Removed slice user-107.slice.
Jul 23 16:53:51 siegeengine /etc/gdm/Xsession[947]: Window manager warning: Log level 16: value "-8.000000" of type `gfloat' is invalid or out of range for property `width' of type `gfloat'
Jul 23 16:53:51 siegeengine /etc/gdm/Xsession[947]: Window manager warning: Log level 16: value "-8.000000" of type `gfloat' is invalid or out of range for property `height' of type `gfloat'
Jul 23 16:53:53 siegeengine /etc/gdm/Xsession[947]: Window manager warning: Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x2600052 (New Tab - )
Jul 23 16:53:53 siegeengine /etc/gdm/Xsession[947]: Window manager warning: meta_window_activate called by a pager with a 0 timestamp; the pager needs to be fixed.
Jul 23 16:54:53 siegeengine gdm[561]: GdmManager: could not fetch uid of session 'c1': No such file or directory
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!   Failed to open reauthentication channel
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     message = '"GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: No sessions for mark available for reauthentication"'
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     fileName = '"/usr/share/gnome-shell/js/gdm/util.js"'
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     lineNumber = '254'
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     stack = '"0 anonymous("result" = [object GObject_Object], "client" = [object GObject_Object])@/usr/share/gnome-shell/js/gdm/util.js:254
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: 1 wrapper([object GObject_Object], [object GObject_Object])@/usr/share/gjs-1.0/lang.js:213
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: "'
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!   Exception was: TypeError: this._userVerifier is undefined
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     message = '"this._userVerifier is undefined"'
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     fileName = '"/usr/share/gnome-shell/js/gdm/util.js"'
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     lineNumber = '436'
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: JS ERROR: !!!     stack = '"(false)@/usr/share/gnome-shell/js/gdm/util.js:436
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: wrapper(false)@/usr/share/gjs-1.0/lang.js:213
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: ("Failed to open reauthentication channel",[object GLib_Error])@/usr/share/gnome-shell/js/gdm/util.js:249
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: wrapper("Failed to open reauthentication channel",[object GLib_Error])@/usr/share/gjs-1.0/lang.js:213
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: ([object GObject_Object],[object GObject_Object])@/usr/share/gnome-shell/js/gdm/util.js:265
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: wrapper([object GObject_Object],[object GObject_Object])@/usr/share/gjs-1.0/lang.js:213
Jul 23 16:54:53 siegeengine /etc/gdm/Xsession[947]: "'

This looks to be a regression introduced in 205

Thanks!