can't connect to XDMCP server with default settings, while Xephyr connects normally

[Norbert]

Steps to reproduce:
1. Configure XDMCP server by adding three lines

[XDMCPServer]
enabled=true
port=177

into /etc/lightdm/lightdm.conf . Then reboot it.
2. On client (18.04 LTS or newer) install remmina-plugin-xdmcp package.
3. Open Remmina, click +, select XDMCP, provide IP address or hostname

Expected results:
* connection is successful

Actual results:
* connection failed, below is the terminal output

$ remmina
Load modules from /usr/lib/x86_64-linux-gnu/remmina/plugins
Remmina plugin glibsecret (type=Secret) has been registered, but is not yet initialized/activated. The initialization order is 2000.
The glibsecret secret plugin has been initialized and it will be your default secret plugin
StatusNotifier/Appindicator support: your desktop does support it and libappindicator is compiled in Remmina. Good.

(org.remmina.Remmina:3492): Gtk-WARNING **: 18:00:24.372: gtk_menu_attach_to_widget(): menu already attached to GtkMenuItem
text:
def: (null)
text: (null)
def: (null)
text: 2600 (Windows XP), 7601 (Windows Vista/7), 9600 (Windows 8 and newer)
def: (null)

text: (null)
def: (null)
(EE)
Fatal server error:
(EE) X11 error
Error code: 8
Sequence number: 5
Major code: 1 Minor code: 0
Error value: 60825008
(EE)

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: remmina-plugin-xdmcp 1.4.11+dfsg-2ubuntu2
ProcVersionSignature: Ubuntu 5.11.0-41.45-generic 5.11.22
Uname: Linux 5.11.0-41-generic x86_64
ApportVersion: 2.20.11-0ubuntu65.4
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: MATE
Date: Thu Jan 27 18:00:54 2022
InstallationDate: Installed on 2022-01-02 (25 days ago)
InstallationMedia: Ubuntu-MATE 21.04 "Hirsute Hippo" - Release amd64 (20210420)
SourcePackage: remmina
UpgradeStatus: No upgrade log present (probably fresh install)