This problem is common if you use 3. part programs when using Word.
If you for an example use an Exel diagram or a formula in Microsoft Equation Editer, this might very well be the source to your problems.
I usually find the corrupt part - and delete it. Then it works again.
To find a corrupt element, click on all of the mentioned above cases. If you get an error back, then this is it.
This is my own experience and its very likely not to be the problem in your case, just thought you should know.