Is this happening with only the one document or all .docx documents? If it is the former then there may well be something wonky with the one document the converter cannot handle which is known to happen occasionally.
I've now received another document from the same sender and it has opened, albeit not with all the formatting intact, so I'm assuming the problems are originating there.
However, we must have fixed something along the way as the document that had been opening, then decided not to, has started opening again.
I'm going to focus on the sender and, if I learn anything, I'll post.
