When this happened to me, I'd notice there was still a copy of word.exe
shown in Task Manager's Processes listing. I had already exited Word but
there was still a copy left in memory (i.e., its window closed but its
process never quite got killed). This doesn't just screw up using Word as
the editor in Outlook but can also cause problems when you reopen what you
think is a new instance of Word. So check Task Manager under its Processes
panel to see if word.exe is still running (after you supposedly exited from
the last window for it); if so, kill it. Then try again in Outlook to use
Word as the editor (although you might have to also restart Outlook). I
haven't any articles explaining my word.exe (and outlook.exe) sometimes
don't complete their process kill when exited.