I appear to be having the same problem as the OP. The only way for me select more than one unit at a time is to hold the Ctrl button and select units from the bottom of the HUD. I can't do it by selecting units on the battlefield no matter where about on the unit I click, I also can't select any units by clicking and dragging to make a box around several units.
Should i report this through the in game bug report mechanism as well?