Can't get fingerprint login to work on my ThinkPad

Hi Geekos!

I own a rather recent Lenovo ThinkPad 13 where I happen to be unable to log in using the built in fingerprint reader (on TW, Gnome). I got *fprintd, libfprint *and fprintd-pam installed from the repos. I used Gnome’s user menu to register my fingerprint, and it finished with a green tick and “finished”, not indicating any wronggoing.
My fingerprint reader is a Validity VFS5011 which should be well supported on Linux:

# lsusb -v
Bus 001 Device 004: ID 138a:0011 Validity Sensors, Inc. VFS5011 Fingerprint Reader
Couldn't open device, some information will be missing
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass          255 Vendor Specific Class
  bDeviceSubClass        17 
  bDeviceProtocol       255 
  bMaxPacketSize0         8
  idVendor           0x138a Validity Sensors, Inc.
  idProduct          0x0011 VFS5011 Fingerprint Reader
  bcdDevice            0.78
  iManufacturer           0 
  iProduct                0 
  iSerial                 1 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           46
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              100mA

On GitHub, all I could find is that the code should be in upstream.
My fingerprint-authentication should be enabled in gdm:

# gsettings list-recursively org.gnome.login-screen
org.gnome.login-screen fallback-logo ''
org.gnome.login-screen disable-user-list false
org.gnome.login-screen allowed-failures 3
org.gnome.login-screen enable-smartcard-authentication true
org.gnome.login-screen banner-message-enable false
org.gnome.login-screen enable-password-authentication true
org.gnome.login-screen disable-restart-buttons false
org.gnome.login-screen logo '/usr/share/gdm/greeter/images/distributor.svg'
org.gnome.login-screen enable-fingerprint-authentication true
org.gnome.login-screen banner-message-text ''

However, if I swipe my finger over the reader on the gdm login page, nothing happens. Even a query in logind for either “finger” or “fprint” has zero results.

What am I doing wrong? Am I forgetting something?

In the meantime, I learned that at least in some occasions, an encrypted /home directory might be a difficulty to set up fprintd. Does anyone know details?
At least, if I log in, and lock the screen, try to log in again, I get a journalctl error message:

gdm-fingerprint pam_warn(gdm-fingerprint:auth): function=[pam_sm_authenticate] flags=0 service=[gdm-fingerprint] terminal=[/dev/tty2] ruser=<unknown>] rhost=<unknown>]

(to be honest, that is of little help to me).
Can some one here provide further insights?