I just learned how to start the system in (formerly) runlevel 3.
From further Suse versions I’m used to having the X started with the command startx.
But now it doesn’t run as naturally as before, since I get the message that the file ~/.serverauth.2084 is missing.
How has this file to look like?
No, it is not missing.
But a normal user doesn’t have the permissions to create it.
It is not possible to run “startx” as normal user since years, it only works for root.
If you insist on doing that anyway, you need to set the Xorg executable suid root.
I.e. uncomment the existing line in /etc/permissions.local and run “chkstat --system” afterwards to “apply” the change.
This is considered to be a security risk though, and that’s why it is disabled by default since years.
Another reason why startx is unsupported since years is that it doesn’t properly register a login session, which means insufficient privileges for the user in other areas too (like accessing the video or sound card, mounting drives, …), so it likely will be necessary to add the user to the corresponding system groups (e.g. video and audio).
Ok, I abstain from using startx 
Thank you.
Note that you can still start display manager (such as sddm, gdm or lightdm) even if you have booted to multi-user.target with;
systemctl start display-manager