After an otherwise uneventful upgrade from 14.10 to 15.04 I cannot login. When I provide my password the screen flickers briefly and I am back at the login screen.
I have checked permissions on .Xauthority and they are set correctly.
I have also tried creating a new user, but the same problem persists.
Before reverting to the login screen I momentarily see an alert box "System program problem detected." I do not know if this is relevant to the issue.
My /var/log/lightdm/x-0.log contains this as the last line:
error setting MTRR (base = 0....) Invalid argument (22)