PDF file changing to a .tmp file when attempting to save changes

I have a user with following issue--  user digitally signs a PDF document.  When he tries to do a 'Save As'  and use the SAME filename,  an error message appears  "The document could not be saved. Cannot save to this filename. Please save the document with a different name or in a different folder."    The original filename is changed to a .tmp file.    We already tried the suggested fix of: -> Preferences, Security (Enhanced)  turning off "Enable Protected Mode at Startup."   Removing and reinstalling the application doesn't seem to help.   User states this error message never appeared in the past but only recently.  (He was able to Save As to the SAME filename in the past after adding a digital signature.)

Below is the version of Adobe being used:

Continuous release | Adobe Acrobat version 2025.001.20756 | 64 bit

Please suggest of any way to fix this issue.