Office client opens files on SharePoint (as Network Drive) as they are stored locally

Hello there,

in our organization we have SharePoint on premise mapped as a Network Drive in every Windows machine. Of course we know it isn't the optimal configuration, in fact every now and then we encounter issues with file names too long, big size videos not streaming, etc., but it's still usable.

It's been two days that a couple of users have this problem:

when I try to open a file stored on SharePoint for the first time, the files opens, and I'm able to edit it.

When I save the file, a little prompt pops up saying "Saving file xxx to local machine..." with a loading bar, like it's saving the file on the local PC, and not on SharePoint. In fact, my collegue noticed that the save icon doesn't have the circle arrows it has when a file is saved on SharePoint.

To be clear:

this is the icon you have when you open a file on SharePoint and everything works fine. It saves and updates the file on SharePoint. When you hover the mouse on the save icon, it tells you that it updates and merge the edits with the other authors.

This one, without the arrows, is the one you have when you open a local file. When you hover the mouse on the save icon, it just tells you that it saves the file.

Well, opening an Office file on SharePoint from those machines, shows the second one, like it's a local file. When I save the first time, the previous prompt appears, if I try to save once again a message saying "Document not found" appears. I then close the file, try to open it again and a message saying "Impossible to open the file" appears and then it suggests me the probable causes, like "File name too long", but I know it's not that the problem because I'm able to open the file and edit it normally on my machine. I did still tried to shorter the file name and opening from a shorter path but it didn't work.

So it seems like Office is retrieving the file from SharePoint for the first time, but then it saves it on the local machine (in the temp folder? who knows). Then I don't know what it actually tries to do...

Also, after the previous actions, if I try to open the same file from my PC, it says that is locked because is opened by the other user, so it doesn't even close the file correctly.

We have SharePoint as a trusted site on all machines (GPO), and the access in the Internet Options is set to current username and password, because all machines are in domain. This is our standard configuration, which works on all machines.

I tried to delete Windows temp folder, Office temp folder. I also deleted local files which had the same names as the ones on SharePoint, so that it doesn't try to open a local version of it (yes, sometimes it happens, and it usually solves the issue).

I'm running out of options, do you have any ideas?