I open Gnome Boxes, press the + button and click Ubuntu Server. I then tells me "Failed to Download" and nothing happens.
Console output (last line states the error. I'm guessing wrong download link):
$ gnome-boxes
(gnome-boxes:12317): Gtk-WARNING **: 21:20:11.150: GtkFlowBox with a model will ignore sort and filter functions
(gnome-boxes:12317): Gtk-WARNING **: 21:20:11.151: GtkListBox with a model will ignore sort and filter functions
(gnome-boxes:12317): Boxes-WARNING **: 21:20:11.505: libvirt-machine.vala:275: Failed to connection to system libvirt: Unable to open qemu+unix:///system: Failed to connect socket to '/var/run/libvirt/libvirt-sock-ro': No such file or directory
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:11.564: g_file_new_for_uri: assertion 'uri != NULL' failed
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:11.564: g_file_get_basename: assertion 'G_IS_FILE (file)' failed
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:11.566: g_file_new_for_uri: assertion 'uri != NULL' failed
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:11.566: g_file_get_basename: assertion 'G_IS_FILE (file)' failed
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:16.204: g_file_new_for_uri: assertion 'uri != NULL' failed
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:16.204: g_file_get_basename: assertion 'G_IS_FILE (file)' failed
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:16.210: g_file_new_for_uri: assertion 'uri != NULL' failed
(gnome-boxes:12317): GLib-GIO-CRITICAL **: 21:20:16.210: g_file_get_basename: assertion 'G_IS_FILE (file)' failed
(gnome-boxes:12317): Gtk-WARNING **: 21:20:17.605: Calling org.gnome.SessionManager.Inhibit failed: GDBus.Error:org.gnome.SessionManager.GeneralError: Invalid inhibit flags
(gnome-boxes:12317): Boxes-WARNING **: 21:20:17.992: downloads-hub.vala:186: Not Found
ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gnome-boxes 3.36.5-0ubuntu2
ProcVersionSignature: Ubuntu 5.8.0-55.62~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-55-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Sat Jun 19 21:20:49 2021
InstallationDate: Installed on 2021-05-31 (19 days ago)
InstallationMedia: Ubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
SourcePackage: gnome-boxes
UpgradeStatus: No upgrade log present (probably fresh install)
Status changed to 'Confirmed' because the bug affects multiple users.