This problem must have a proper solution because the message must be intended and is obviously built into the MS Word product. For others who have come across it and cannot find a fix, here is a slow way I have found a way to fix the problem - but it is tedious and needs to be done one file at a time. In my case, from a collection of perhaps 50 files, about a dozen have this problem. All the files are very similar as regards styles and other formatting, so using a file that does not have the problem I take a copy and delete all the text. This leaves a template because styles etc remain without the text that uses them. Then the damaged file is stripped of all the formatting, leaving only undifferentiated text. This is then transferred into the empty template. The files are mostly text so the inherited styles are used to reconstruct the damaged file. When loaded this new file is free of whatever was causing the error. For anyone still looking for the reason, it began (or was noticed) soon after upgrading my OS from Mojave to Monterey.
Another problem with the same files has occurred - so just to notify everyone of the symptoms: When typing text in a perfectly normal manner, for no apparent reason, any key produces a ringing sound and no character is transferred. The cursor flashes at that point and there is no way of moving on, other than grabbing the mouse and moving its cursor over the text positioning cursor and clicking, thus allowing progress to resume. Sometimes the fault occurs again several times in a paragraph, other times only once in a session.
