Equation Editor "A file error has occured"

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This problem occurs in Word 2007 when using equation editor.
Occasionally I will be working on a document containing lots of equations made in equation editor. I decide to save and an error saying "A file error has occured" appears, and the document refuses to save.

This is a very widespread problem. A quick Google search brings up many unresolved threads on different forums, all concerning this issue. e.g.:

• http://help.wugnet.com/office/Word-s...ct1126389.html
• http://www.thestudentroom.co.uk/showthread.php?t=729493
• http://www.eggheadcafe.com/software/...-occurred.aspx
• http://www.pcreview.co.uk/forums/wor...-t3956212.html

The following previously suggested "solutions" do not work:

1. Cutting and pasting the formulas into a new document
2. Accepting all changes to document in the review tab
3. Using 'open and repair'
4. Systematically cutting formulas away and trying to save, in order to find out which formula is causing the problem. (I don't know why this won't work for me)