If X wouldn’t work, you wouldn’t see that circle or a mouse pointer either… 
But yes, something goes wrong there during login:
The X11 connection broke (error 1). Did the X11 server die?
kdeinit5: Fatal IO error: client killed
kdeinit5: sending SIGHUP to children.
klauncher: Exiting on signal 1
The X11 connection broke (error 1). Did the X11 server die?
kdeinit5: sending SIGTERM to children.
kdeinit5: Exit.
Qt: Session management error: IO error occured opening connection
Unexpected response from KInit (response = 0).
QProcess: Destroyed while process ("kwin_x11") is still running.
startkde: Could not start ksmserver. Check your installation.
Error: Can't open display: :0
startkde: Shutting down...
If it works for other users, it must be something user specific though…
Maybe try to delete everything in /tmp/ and /var/tmp/ (as root). Some temporary files for your user might have wrong permissions.
The last thing before that are messages from kscreen. So maybe try to delete ~/.local/share/kscreen/ and see if it helps.
Yes. I’m using Enlightenment at the moment. KDE Plasma and Plasma 5 do not work at least.
Ok, that would rule out a problem with ~/.Xauthority at least.
Btw, “KDE Plasma Workspace” and “Plasma 5” are exactly the same thing. The former is only included for compatibility with previous versions (to prevent problems when upgrading).
Yes. Actually I changed /home/somename to /home/somename_old. Tried to delete the account with YaST but it didn’t like this approach so I had to add empty /home/somename to make YaST happy. Then deleted the account and created it again. Tried to log in with Plasma but didn’t work. Then deleted the newly created home dir and moved the old back (to get settings etc. in order). This part makes this problem strange. I thought that under home dir there are KDE/Plasma settings and that’s it but apparently that is not the full case. There are settings somewhere else.
No there aren’t. Except the system-wide defaults, but they again apply to all users.
If the problem does occur after cleaning out $HOME, I still think it’s most likely a permission issue.
Please install Mesa-demo-x and post the output of “glxinfo | grep render”.
Another idea: try running “startkde” in IceWM. Does the Plasma desktop appear?
Please post the output of:
ls -ld /home/*username*
(proper account name replaced with xxxx)
drwxr-xr-x 42 xxxx users 4096 Aug 2 10:46 /home/xxxx
If both xxxx are really exactly the same, this is ok.