It's working again... And I did not reset ACAD, nor make any changes with system variables or other settings.

 I just started up the iMac this morning (after being shut down overnight). Opened AutoCAD LT 2015. Opened same file as before and began using tools; Stretch, Scale, Copy,..etc., even select with cursor. Shift-Click deselect worked every time. Amazing, how it works... and then it doesn't work.. and then it works again.