Cannot save attachment in Documents

I'm helping Frank with a perplexing problem; he cannot save attachments in his own "Documents" folder.

Background: When he migrated from Windows7 to Windows10 he got converted to logging on with his Microsoft Account. We neither fully understand this. Frank has only one Windows system, so I created a local user "Frank" for him. This is probably where the access problem started.

I copied his Documents folder to Documents_New, renamed Documents to Documents_Old, and renamed Documents_New to Documents. At this point, Windows explorer stopped showing Documents, instead showing Documents_Old.

His problem saving attachments persisted, if he used the Documents_Old folder (naturally). If he navigates to C:\Users\Frank\Documents to save his attachment, it works, but it is cumbersome.

When I used a CMD prompt to inspect the folders, I found two under C:\Users\Frank - Documents_Old and "Documents - copy" (or something like that). This is where things get interesting...

I created a file in "Documents - copy" using "Echo Hello > test.txt". It worked. So I tried that against Documents_Old, and that worked as well. So Frank *does* have create privilege on the original (renamed) Documents folder.

But he still cannot save documents to into the original Documents folder (now named Documents_Old).
He is using Thunderbird, but the problem also occurs if he edits a txt file in Documents_Old then tries to Save As into Documents_Old. So it's probably not an issue with the programs that he uses for email and editing.

At this point, I'm nearly out of ideas. I'm going to ask him to run his Thunderbird as Administrator, just to gather more facts. I expect it will work, but it's probably not wise to run that way permanently.

Can anyone shed light on what might be going on here, and offer a fix?

Have a look on the location tab of the properties and see if it is pointing to the correct place... Perhaps it is pointing to "Documents_old"

Right click on "Documents" then take the "Location" tab like this...

I can't get access to Frank's PC for a couple of weeks. But the problem (as I see it) is:

Why cannot Thunderbird save into Documents_Old (which was his original "Documents" folder, as seen in explorer.

Earlier this morning I had a flash of inspiration... C:\users\frank contains no folder "Documents" any longer. If Thunderbird is trying to save in C:\users\frank\documents, then that might be the cause of the problem. If so, then a more accurate Thunderbird error message would have this fixed in a trice.

Anyway, next time I get some hands-on time on Frank's PC, I'll create a C:\users\frank\documents folder, whilst logged on as Frank, and see what happens.

In the meantime, is there any documentation I could read on the relationship between the "Documents" folder as seen in explorer, and how that maps to the actual physical folder used, as seen through a CMD prompt?

I've seen the location information in the properties tab of Franks document folders, and it looks OK to me.

This is the nub of the problem... everything looks OK to me (and I used to be a Microsoft Support person in IBM a few years ago), it just isn't working OK!