I created a new user in Yast2 without checking the "Defaults for New Users" as
i've created users in the past with no issues. I've not created a user for a
few years so i have no idea when this became a problem. (This has already been
logged as a separate issue)

Process:
I tried to log in with this new user (ssdm) and immediately after entering the
password and hitting <enter>, I got an Xsession error message (looked like an
old 1980s format) saying its disabled and an option to press an "Okay" button.
This "Okay" button is unclickable and un-escapeable. 

Solution
The only way to clear that Xsession error is to Ctrl Backspace Backspace.

Reason for disabled login.
I then went back and checked the list of users in Yast2 and my new one did not
belong to any groups. I checked "Defaults for New Users" and saw "users" was
the default group but the "Default Login Shell" and "Skeleton for Home
Directory" were blank.  I put in /usr/bin/bash and  /etc/skel, deleted the old
user and created a new one which allowed me to login fine. 

opensuse:tumbleweed:20210928
Qt: 5.15.2 KDE Frameworks: 5.86.0 - KDE Plasma:  5.22.5 - kwin 5.22.5
kmail2 5.18.1 (21.08.1) - akonadiserver 5.18.1 (21.08.1) - Kernel: 
5.14.6-1-default  - xf86-video-nouveau:  1.0.17