[X2Go-User] X2Go, Debian, Openbox - How run graphical clients as a simple user

[Lmhelp1]

Hello,

I am using X2Go under Debian with Openbox.
I have upgraded to Debian Buster, I didn't use to experiment the problem 
below with Debian Stretch.
My problem is about running graphical clients (like xeyes, xterm, gvim, 
etc.) from a console logged in either as a "simple" user ("user_2" 
below) or as "root".
"user_2" cannot run these clients, "root" can.
I would like both of them to be able to run these clients.
Below, is what happens ("user_1" is the user that started the X2Go 
session, it is also a "simple" user).

user_1> su user_2
Password:
No protocol specified
xrdb: Resource temporarily unavailable
xrdb: Can't open display ':50'

user_2> exit

user_1> su
Password:
root> xeyes
<OK>

Can you tell me how to allow "user_1" to run graphical clients like 
xeyes, xterm, gvim, etc.?

Best regards,
--
Léa