PowerPoint 2011 won't open files after Mac upgrade

I upgraded from an old MacBook Pro to a spiffy new one. Migration went without incident but now, PowerPoint (Office 2011) won't open any files.

I did the steps to re-register the Microsoft Office package (entering activation code) and ran the Office updater to get the latest. Word and Excel work normally. But PowerPoint will not open any files -- I tried many different /pptx files. Same if I double click a pptx file, or if I try to open using the file dialog. PowerPoint is running but no window opens. And "New presentation" from the File menu also brings up no window.

Any ideas?