Did you have Outlook running?
The link between Word and Outlook can be a bit fuzzy at the best of times. It is invariably better if Outlook is already running. This is the first thing to try.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com

AS my Outlook's always on, this was not the solution. It fixed itself when I let Office 365 update. Hopefully the problem won't recur. Thanks anyway.