Cause?: Selected track/group can not be changed

Out of the blue:
Open project. Click on a track/group/midi/fx (any kind of) track and that track is not selected because it reverts/jumps back to the track that was already “selected” when the projected was opened.
True for all projects.
Delete: User/**/AppData/Roaming/sSteinberg/Cubase8_64 - “Defaults.xlm”…problem gone.
Obviously something is corrupt in the defaults but does anyone have a clue as to the cause of this problem?