Word for Mac 2019 on Big Sur won't save doc file

When I try to save a Word document, the Save command is accepted but the file is not saved. There are no error messages. The Finder shows the previous save date and time.

If I Close the doc window, a dialog box appears with a prompt to save the file, the window closes, and then reopens. The document is unsaved and the Finder shows the previous save date and time.

If I try to Save As with a new file name, the Finder shows the new file briefly (zero bytes file size) and then disappears from the list.

If I try to Quit Word, a dialog box appears that with a prompt to Save, clicking Save produces a second dialog box that says the location of the document cannot be determined.

I'm running Word for Mac 2019 (16.43) on Big Sur (11.0.1). The application and (original) file are both located on the computer's hard disk. Both programs appear to be the latest revs. I suspect it's a Big Sur compatibility issue since it didn't happen on Catalina.

Any suggests to recover my edits before I loose hours of work? Thank you.