This error started occurring around May 13th for both me (iMac 21.5 2009) and my friend (Macbook Pro 2013), both of us running Outlook for Mac 2011 (14.5.0) under Yosemite (10.10.3).

A click-red-ball-close behaves normally, leaving Outlook active; however, after that, a normal click-icon-open has no effect -- no animation, no nothing. We have to right-click-icon-quit to deactivate Outlook. Then a click-icon-open invokes the animation and a good open.

Does this look familiar to anybody? It's too much of a coincidence to ignore our 15-miles-apart systems having the same problem start at the same time.