A family of Microsoft word processing software products for creating web, email, and print documents.
Do the affected documents have anything in common, as far as you can tell? For example, are they in some other file format (that is, not in the default *.docx format)?
I should add that this is a "classic" error message. In the past, it has been suggested that the error might be caused by antivirus software flagging certain file types (see for example https://answers.microsoft.com/en-us/msoffice/forum/all/error-word-cannot-start-the-converter-wordperfect/3535a185-129d-43a7-b38e-35d3cb44ff73).