In addition, I installed new test servers Debian 9 and Debian 10

in Debian 10 everything works well, but in Debian 9 the same errors
described earlier

Operating System: Debian GNU/Linux 9 (stretch)
Kernel: Linux 4.9.0-12-amd64

but if update Debian 9 OpenSSH_from version 7.4p18 to 8.1p1 then everything
starts to work

x2goserver/oldstable,now 4.1.0.3-0x2go1+git20191115.1708+9.main.1 amd64
[installed]
x2goserver-common/oldstable,now 4.1.0.3-0x2go1+git20191115.1708+9.main.1
amd64 [installed,automatic]
x2goserver-extensions/oldstable,now
4.1.0.3-0x2go1+git20191115.1708+9.main.1 all [installed,automatic]
x2goserver-fmbindings/oldstable,now
4.1.0.3-0x2go1+git20191115.1708+9.main.1 all [installed,automatic]
x2goserver-printing/oldstable,now 4.1.0.3-0x2go1+git20191115.1708+9.main.1
all [installed,automatic]
x2goserver-x2goagent/oldstable,now 4.1.0.3-0x2go1+git20191115.1708+9.main.1
amd64 [installed,automatic]
x2goserver-xsession/oldstable,now 4.1.0.3-0x2go1+git20191115.1708+9.main.1
all [installed,automatic]