Black screen on graphical target X11 after update/upgrade/reboot during boot

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Environment

SUSE Linux Enterprise Server 12 Service Pack 2 (SLES 12 SP2)
SUSE Linux Enterprise Server 12 Service Pack 3 (SLES 12 SP3)

Situation

After upgrading from e.g. SLES12SP1 the graphical target shows black screen.
Starting X11 manually e.g. with startx works, changing display manager to xdm and reboot server works, too.

Resolution

Change user IDs to be unique.

Cause

Duplicate UIDs which cause the gdm account service to hang.

Disclaimer

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