WMS errors are hard to diagnose but often are the result of some add-in to Outlook that is not behaving properly. There is a utility called KnockOut that sits in the system tray and with a click of the mouse will force Outlook to close completely, even if some add-in is trying to keep it open. The KnockOut icon gives visible confirmation if Outlook has in fact completely shut down or not so you always know when to use it.
It's free so I would recommend you try it and see if it will help your shut-down situation.
http://sunflowerhead.com/software/knockout/