It would not boot into sddm, but would allow me to start sddm a root from the command line in ctr-alt-F1 screen.
I spend many hours researching .........
Here is the fix I found out myself eventually and would like to share. This problem and the solution was replicable on both boxes:
Insert a requirement to wait for Plymouth into sddm.service like this:
...and then stop and start sddm again with systemctl start sddm.service.[Unit]
Description=Simple Desktop Display Manager
There is still a problem with broadwells and sddm crashing when the screen power-saves. This is apparently related to a known issue at kdeplasma. hope they fix it soon.