U2F on Qubes

[SP]

Hello,

I followed the instructions here (https://www.qubes-os.org/news/2018/09/11/qubes-u2f-proxy/) to setup the U2F proxy:

- I didn't install anything on sys-usb

- I installed `qubes-u2f` in my broswer VMs

- I installed `qubes-u2f-dom0` in dom0 and enabled the `qubes-u2f-proxy` service for my browser VMs

- In dom0 I have the default policy for U2F (not per-qube access)

I see that the qubes-u2f-proxy service is running on the browser VMs where I want to use U2F. However, when I try to use U2F in Google Chrome, I see that `qrexec` returns a 127 status:

```

● qubes-u...@sys-usb.service - U2F proxy for sys-usb

   Loaded: loaded (/lib/systemd/system/qubes-u2fproxy@.service; enabled; vendor preset: enabled)

   Active: active (running) since Sun 2019-09-22 16:54:54 PDT; 13min ago

 Main PID: 426 (qu2f-proxy)

    Tasks: 5 (limit: 4915)

   Memory: 27.4M

   CGroup: /system.slice/system-qubes\x2du2fproxy.slice/qubes-u...@sys-usb.service

           └─426 /usr/bin/python3 /usr/bin/qu2f-proxy sys-usb

Sep 22 16:54:54 primary systemd[1]: Starting U2F proxy for sys-usb...

Sep 22 16:54:54 primary systemd[1]: Started U2F proxy for sys-usb.

Sep 22 17:08:37 primary qu2f-proxy[426]: 2019-09-22 17:08:37,887 U2FHIDQrexecDevice.qrexec qrexec_client.returncode=127

```

I am not sure why this is happening. Any pointers on how I should debug this / what logs I should look at would be very helpful.

Thanks!