A family of Microsoft word processing software products for creating web, email, and print documents.
It appears that you have carried out most of the troubleshooting steps already that I would suggest.
It’s appears that the maybe the documents from older version of Word are the cause and could be corrupted.
Does the issue happen when using newer type documents?
If not, it maybe a case of opening the old Word documents in another document editing tool such as Wordpad or other and copying the contents over to a newer type document.
It’s not an ideal situation but it may be a work around.
I hope this information helps. If you have any questions, please let me know and I'll be glad to assist you further or If you find it helpful, you can mark this comment as the answer.
Kind regards
Tim