Upgrade vom 20.3 to 21.3 fails

[MartinPabela]

After passing all steps before ...
Running check 'Phase 3: Aktualisierung'

APT Repositories:

Repos: No active apt repos in: /etc/apt/sources.list
No active apt repos in: /etc/apt/sources.list.d/home:alex_sh:gsmartcontrol:stable_latest.list
No active apt repos in: /etc/apt/sources.list.d/jtaylor-keepass-focal.list
Active apt repos in: /etc/apt/sources.list.d/official-package-repositories.list
1: deb http://linux-mint.froonix.org vanessa main upstream import backport
2: deb http://mirror.23media.com/ubuntu jammy main restricted universe multiverse
3: deb http://mirror.23media.com/ubuntu jammy-updates main restricted universe multiverse
4: deb http://mirror.23media.com/ubuntu jammy-backports main restricted universe multiverse
5: deb http://security.ubuntu.com/ubuntu/ jammy-security main restricted universe multiverse
No active apt repos in: /etc/apt/sources.list.d/syncthing.list

... the upgrade dies in ...

Running check 'Sitzungssperrung'

in the GUI I see:
Traceback (most recent call last):
File "/usr/lib/linuxmint/mintupgrade/checks.py", line 94, in run
self.do_run()
File "/usr/lib/linuxmint/mintupgrade/checks.py", line 832, in do_run
subprocess.Popen(cmd)
File "/usr/lib/python3.8/subprocess.py", line 858, in init
self._execute_child(args, executable, preexec_fn, close_fds,
File "/usr/lib/python3.8/subprocess.py", line 1639, in _execute_child
self.pid = _posixsubprocess.fork_exec(
TypeError: expected str, bytes or os.PathLike object, not NoneType

My terminal is a terminal of root, the X runs as a user, which has allowed root to start some X session using xhost command

[MartinPabela]

Additional info: mintupgrade is version 2024.01.12

[terr72]

My terminal is a terminal of root, the X runs as a user, which has allowed root to start some X session using xhost command

That's the culprit. You should run a sudo mintupgrade as user, not root.
This should IMHO however be caught earlier in the process by mintupgrade - or at least be shown in a way that's easier to understand.