Hi,
This is problem that has popped up since Word 2007. Here is a page describing both the cause and solution:
Page numbering /header / footer problems Word 2007 - 2016
(http://www.addbalance.com/word/pagenumber10.htm)
Basically, it is a corrupt built-in building blocks file and is easily fixed.
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This link is to a page on my website. It is safe. I am providing it because I think it contains information that will help you. However, as an Independent Advisor I am required to add the following when posting here with a link to any non-Microsoft site:
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Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised
on the site before you decide to download and install it.
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I hope this information helps. Please let me know if you have any more questions or require further help.
Regards