MS Word will not save word .docx document after editing - Mac OS Catalina, Word 16.43

Hi. I have been using MS Word for Mac for years, don't usually have any problems. Currently version 16.43 (Build 20110804) running on Catalina 10.15.7.

A .docx document I have downloaded and edited refuses to save to any local disk or one-drive. As far as I can see it is not write protected and Word has write access to the disks. Autosave will not save to any location I set for it either. The 'save as' creates an empty 0 bytes document with any name/directory I set but says it cannot save the document there. The "do you want to save your changes" when trying to close Word has the same problem. Anyone else had this (I believe a great many people have)?

Any help much appreciated.