After installing the key for SLES 12 on our DELL PowerEdge R320, I got some updates for X11:

# 2015-03-27 08:00:47 xorg-x11-server-7.6_1.15.2-21.1.x86_64.rpm installed ok
# Additional rpm output:
# Updating /etc/sysconfig/displaymanager...

After this, it seems that I can't log in to gdm, because the screen turns black after the green running dots have been running 8 times.

I changed /etc/permissions.local to be able to run startx after [Ctrl]+[Alt]+F1, but it is still the same black screen even if I restart the display-manager.

If I change DISPLAYMANAGER to "kdm" in the /etc/sysconfig/displaymanager you get the grey SUSE-background, and a system error window in the lower left corner, but you can't actually access anything at all.

The only thing in the ~/.xsession-error-logs is:
gnome-session-is-accelerated: llvmpipe detected.

The only error I have seen in the Xorg.n.log is:
[ 2144.993] (EE) mga: The PCI device 0x534 at 06@00:00:0 has a kernel module claiming it.
[ 2144.993] (EE) mga: This driver cannot operate until it has been unloaded.

No files in the /etc/X11/xorg.conf.d/-folder has been changed/updated.

The xdm status is up-and-running:
display-manager.service - X Display Manager
Loaded: loaded (/usr/lib/systemd/system/display-manager.service; enabled)
Active: active (running) since Mon 2015-04-06 17:12:51 CEST; 58min ago
Process: 3364 ExecStop=/usr/lib/X11/display-manager stop (code=exited, status=0/SUCCESS)
Process: 3663 ExecReload=/usr/lib/X11/display-manager reload (code=exited, status=0/SUCCESS)
Process: 3377 ExecStart=/usr/lib/X11/display-manager start (code=exited, status=0/SUCCESS)
Main PID: 3390 (gdm)
CGroup: /system.slice/display-manager.service
├─3390 /usr/sbin/gdm
└─3396 /usr/bin/Xorg :0 -background none -noreset -verbose 3 -logfile /dev/null -auth /run/gdm/auth-for-gdm-FpAmt3/database -seat seat0 vt7

We did a test with enabling VNC, but that did not help. It is black screen on VNC also.

I have tried booting with both nomodeset and failsafe, but no luck, nothing else than the black screen...

I am out of ideas right now - someone who can point me in the right direction of further analysis?