I solved my Save problem by deleting one formula and illustration at a time to find out which one was causing the document to be corrupt, and then I redraw the offending illustration, and I could save it.
Since that day, I have worked a lot with my documents, and almost once a day, at least one file gets corrupt and I cannot save it. [I cannot believe that Word is this buggy, but apparently I am the only one using Word to write long, technical, documents, and thus the only one to see these bugs.]
Anyhow, I can live with this. I have learned to Ctrl+S once a minute, and when it fails, I simply close without saving and open the last saved version of the document, and rewrite the new material I wrote the last minute.
But now I have an even worse problem: One of my long documents won't even open!
The dialog says, "The file %s cannot be opened since there is a problem with its content. Information: The name in the ending tag of the element has to match the element type of the starting tag. Place %s Part %s,, Row: %d,, Column %d"
The document is more than 40 pages long, and I have spent many weeks working on it. It contains a lot of formulae, illustrations, footnotes, bibliography, cross references, etc. Don't tell me I have to redo everything. Is there anything I can do?