Unable to login GUI while starting CentOS 7.6

[trueman]

uname -r: 3.10.0-957.1.3.el7.x86_64
Machine: AMD® Athlon(tm) ii p360 dual-core processor × 2 on board of a HP 625 Laptop

Even though not having been brought up with Unix I‘m quite familiar with several Linux-Distributions (X/K/L-Ubuntu, Manjaro/Arch and CentOS 7.5). And I‘m used to do a clean New-Install when necessary.
Since updating to CentOS 7.6 (1810) I am confronted with the above mentioned issue. And since then I tried several (!) New-Installs on a wiped-out SSD (automated partitioning) with no disturbing surrounding like a different OS with its virtual machine. I tried as well a „Minimal-Install“ adding just openbox but next time a fresh New-Install of a „Development-Workstation“ including an „Office- and Productivity-Suite“ with 'gnome-session', 'gnome-session-common-session' and all that stuff around. Upgrading from CentOS 7.5 (1804) or New-Install / Net-Install from http://mirror.centos.org/centos/7/os/x86_64/ next http://mirrors.sonic.net/centos/7.6.1810/os/x86_64/ last http://mirror.centos.org/centos/7.6.1810/os/x86_64/ , it's always the same: logging in at the console as UserMe and startx (multi-user.target) let me go through into the GUI but neither systemctl [status] / [isolate] / [enable] / [set-default] graphical.target nor editing ~/.xinitrc helped me forward. It leads either to just a flickering black sreen or with some luck it shows a frozen desktop with the typical background but missing all those folders and items you are used to see. After 'systemctl enable graphical.target' asking for the status lets me get to know that the target is 'dead'.