TigerVNC Xfce startup failing on new Debian 12 bookworm install. "Another session manager is already running" Worked on Debian 10

I've set up a temporary Debian 12 instance on a laptop to test with prior to migrating a desktop from Debian 10 to 12. The only thing not working so far is the tigervnc instance, but that's critical. (Yes, this post is long, but I wanted to make sure that anyone viewing it had enough information to go off of, and to minimize the number of follow-up questions) Several years ago I set up tigervnc-standalone-server on a Debian 10 amd64 system with Xfce4 desktop, and it's worked like a champ ever since. Extremely stable, very responsive, etc. However, it's time to move to Debian 12, and for the life of me, I can't get tigervnc to work on the Debian 12 test system that I've set up (installed using debian-12.2.0-amd64-DVD-1.iso) ***The existing Debian 10 configuration, which works fine (see further below for what isn't working):*** ------ **Package version:** Tigervnc-standalone-server 1.9.0+dfsg-3+deb10u3 **Config files:** testuser@system:~$ cat ~/.vnc/xstartup #!/bin/bash xrdb $HOME/.Xresources unset SESSION_MANAGER unset DBUS_SESSION_BUS_ADDRESS startxfce4 (the Xresources below is an empty file, so the xrdb line above probably doesn't do anything) testuser@system:~$ cat ~/.Xresources testuser@system:~$ (the geometry in the config file below is specified in the startup command, so the file below may not matter either) testuser@system:~$ cat ~/.vnc/config geometry=1920x1080 dpi=96 **Successful launch:** testuser@system:~$ vncserver :1 -geometry 1920x1080 -localhost no New 'system.org:1 (testuser)' desktop at :1 on machine system.org Starting applications specified in /home/testuser/.vnc/xstartup Log file is /home/testuser/.vnc/system.org:1.log Use xtigervncviewer -SecurityTypes VncAuth,TLSVnc -passwd /home/testuser/.vnc/passwd system.org:1 to connect to the VNC server. testuser@system:~$ **Usage notes:** If the Debian system is rebooted, I'll login to the testuser account using the kvm, open a terminal, and enter the command above. I'll then logout of the KVM, and from that point on use VNC Viewer (RealVNC) on a Windows machine to connect to the Debian box. After the initial Debian login, I can disconnect and re-connect the VNC client at will without having to logout of the Debian user's desktop (home environment, no port-forwarding, minimal security issues in doing so). Works perfectly. And yes, I know I could configure it to auto-start on boot, but I'd rather leave that part manual. ***The new Debian 12 configuration attempts, none of which work work:*** -------- **Package version:** tigervnc-standalone-server 1.12.0+dfsg-8 $ vncpasswd has been executed, and a password created for the same user which is issues the launch command. When using VNC to start only /usr/bin/xterm, this password is accepted by the remote VNC viewer. **Config files:** ~/.Xresources file: not present (wasn't needed before)
~/.vnc/config file: same as the Debian 10 file (defining geometry and dpi, but geometry is being overridden with the vncserver command)
~/.vnc/xstartup file: have tried multiple versions, none of which correctly start up an Xfce4 desktop on VNC. See below for details.
Below are the different versions of ~/.vnc/xstartup which I've tried, with the corresponding output when attempting to startup the server:
1 Original, duplicating the Debian 10 file: #!/bin/bash unset SESSION_MANAGER unset DBUS_SESSION_BUS_ADDRESS startxfce4 Tailed log output from $ vncserver command: =================== tail /home/sysuser/.vnc/testcase.nodomain.org:1.log =================== /usr/bin/startxfce4: X server already running on display :1 (process:2353): xfce4-session-CRITICAL **: 02:42:27.394: dbus-launch not found, the desktop will not work properly! xfce4-session-Message: 02:42:27.515: SSH authentication agent is already running gpg-agent: a gpg-agent is already running - not starting a new one =========================================================================================== Session startup via '/home/sysuser/.vnc/xstartup' cleanly exited too early (< 3 seconds)! 2 Without DBUS unset: #!/bin/bash unset SESSION_MANAGER startxfce4 Tailed log output from $ vncserver command: =================== tail /home/sysuser/.vnc/testcase.nodomain.org:1.log =================== /usr/bin/startxfce4: X server already running on display :1 xfce4-session-Message: 01:46:16.716: SSH authentication agent is already running gpg-agent: a gpg-agent is already running - not starting a new one =========================================================================================== Session startup via '/home/sysuser/.vnc/xstartup' cleanly exited too early (< 3 seconds)! 3 Without any unsets: #!/bin/bash startxfce4 Tailed log output from $ vncserver command: =================== tail /home/sysuser/.vnc/testcase.nodomain.org:1.log =================== /usr/bin/startxfce4: X server already running on display :1 xfce4-session: Another session manager is already running =========================================================================================== Session startup via '/home/sysuser/.vnc/xstartup' exited with status 1! **Commonalities:** In all cases, the outputs above are followed by this: Maybe try something simple first, e.g., tigervncserver -xstartup /usr/bin/xterm The X session exited with status 1! Killing Xtigervnc process ID xxxx... success! and when I do a $ vncserver -list command, it comes back with no instances running (header with nothing below). **VNC's core is working:** I know the VNC server is working at a base level, since if I reduce the xstartup file to nothing but: #!/bin/bash /usr/bin/xterm then, the $ vncserver -list command will show an X instance, I can connect to it from the Windows viewer, and a terminal window (but nothing else) is presented. So it's just a matter of the Xfce4 not starting correctly. Any ideas??