sddm not starting after upgrade

[Richard Barmann]

I have tried all the suggestions in the bug report but none of them have 
helped me. I have installed all the system updates showing daily on my 
monitor. Then I updated from 14.04 to 14.10. then I tried to install 
15.04 and get the  message 1.4178621 ACPC (I think) PROBE FAILED/ 
Starting Version 219.
I still get the same black screen after logging in and getting to the 
next screen (with a wide band across the center). What can I do to get 
into 15.04?

Thanks for any help
Dick Barmann