Runtime-error '1004' cannot access the file, depending on the computer

Hi all,

The context:
I have an Excel file on a SharePoint server.
People can open it, edit it, save it.
When they save it, there is a macro to save a copy on the network (public drive), and replace the previous copy if it exists :

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The problem:
If I do it from my computer, it always works.
But for some people, I always get this error :

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The file is saved on SharePoint but the copy is not saved.

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I did some researches but did not find anything.