cannot switch to xfce4 - stuck in gdm3

[carmima]

Hello
I think I tried everything:

apt install xfce4, xfce4-goodies
dkpg-reconfigure xfce4
apt remove gdm3
tasksel

nothing works, only the login screen changes.
I still get gnome after login as display manager.

what can I do besides asking for help?
all of this in buster/10

Thank you

[peter_irich]

What is your DM - Display Manager?
When you have greeting on the monitor, can you select session?

Peter.

[carmima]

yesterday after boot I got the xfce4 prompt to enter username and password
the system was in graphical mode, with mouse functionnal.
I tried to log off. I got an empty debian logo back screen where I can read Debian 10.
the screen is empty. no controls whatso ever
now here is what I got after reboot:
same blank screen empty background blue debian 10 logo.
cannot login anymore.
meanwhile I entered apt-get update/upgrade sequence as usual whithout problems
My guess is that I will have to check the system logs to investigate
which log file should I check?

[carmima]

Here is what I did
I opened a terminal with ctrl+alt+F1
sudo dpkg-reconfigure xfce4
got a progress bar
rebooted then I get a working loging prompt
meanwhile I saw passing start messages on the screen
LSB: Starting gnome display manager

now I have a login prompt
it seems to be the xfce4 welcome login with 2 empty lines
username:
Password:

on the top line menu I have some options:
oh dear! I just saw there is a tiny clickable icon which shows many options:
(*) default session
() gnome
() gnome
() xfce4

I am sorry.