I discovered something what looks like bug in vncmanager, or if not bug, then
massive limitation; a limitation not clearly declared in the openSUSE manual:
when vncmanager service is running - users can log in ONLY using VNC, but not
locally.

When vncmanager is running, and I attempt to login locally, GDM starts, shows
standard login screen, with user list, but every login attempt ends with sudden
return to GDM without any explanation and without any traces in logs. Any error
whatsoever anywhere. Any warning, any hit. Just magical return back to GDM. But
at the same time I was able to log in via VNC. It took me a lot of head
scratching to figure out the culprit - vncmanager. When I disable the service,
I was immediately able to login locally.

I really like vncmanager, I wanted persistent sessions and using vncmanager is
an easy and straightforward solution. Well, at least it seemed to be. I set it
us using this great manual:
https://doc.opensuse.org/documentation/leap/reference/html/book.opensuse.reference/cha.vnc.html#vnc.persistent.vncmanager
4.4.2 VNC Session Initiated Using vncmanager

I use my box as a home server, experimental server, mostly accessing it by ssh
or VNC, but it also doubles as my HTPC/Kodi box, because it sits next to my TV;
and I also occasionally log in locally, using TV as monitor. If vncmanager is
used, I cannot use it as Kodi box or log in directly. This means vncmanager
would be a no-go for me and that's a shame.

This issue is also discussed in openSUSE Forum thread:
https://forums.opensuse.org/showthread.php/535175-Bug-in-vncmanager-If-running-no-one-can-log-in-locally.

And I also reported it on the project's GitHub site:
https://github.com/michalsrb/vncmanager/issues/2

How to verify that vncmanager is the driving service behind VNC:

$ ps ax | grep -i vncmanager
 /usr/bin/vncmanager
 /usr/bin/vncmanager-controller --tray --start-setup

$ systemctl status vncmanager
��� vncmanager.service - VNC manager
   Loaded: loaded (/usr/lib/systemd/system/vncmanager.service; enabled; vendor
preset: disabled)
   Active: active (running) since Sun 2019-02-17 21:08:10 GMT; 2h 33min ago
 Main PID: 1177 (vncmanager)
    Tasks: 8 (limit: 4915)
   CGroup: /system.slice/vncmanager.service
           ������1177 /usr/bin/vncmanager
           ������1916 /usr/bin/Xvnc -log *:syslog:30,TcpSocket:syslog:-1 -inetd
-MaxDisconnectionTime=5 -securitytypes=none -displayfd 10 -geometry 102>

$ systemctl status xvnc.socket
��� xvnc.socket - Xvnc Server
   Loaded: loaded (/usr/lib/systemd/system/xvnc.socket; disabled; vendor
preset: disabled)
   Active: inactive (dead)