If you’ve seen a similar notification, I empathize with your pain. I don’t know that there is one solution to this problem, either, so I’m going to share a number of them we’ve used and hope that one (or all) of them will help you.
Basically a file is added through file explorer (a cloud library in OneDrive or SharePoint being synced locally to your computer) but then after a moment a notification appears which says “You now have two copies of a file; we couldn’t merge the changes in [filename]” and then the filename is appended with your computer name again and again until eventually the filename is too long and is harder to delete. Let’s not get to that point.
When working in Office 365 or SharePoint and you open a document for editing, you get two choices. Edit in Word (or Excel) or Edit in Browser. Editing in browser is typically a safe route but it doesn’t give you full functionality like the clients will.
The issue I’m discussing here is when a user tries to edit a document from SharePoint using the client (not Edit in Browser) the client opens to a blank, gray background (no document) or doesn’t open at all. This is likely an account conflict in syncing or accessing.
In other cases, the document may open, but as read-only. If that’s the case, it’s likely permissions-related. You might first check the user’s specific permissions in SharePoint. Sometimes because of broken (not inherited) permissions, or partial access to a site, users are able to edit in browser, but not in the client. If this is your situation, it could well be the cause.
Hopefully this is a simple fix for you, but as it’s become clear to me a number of times with this issue it can be quite complicated. I have a couple fixes, though the second is less ideal. If anyone else has run into this and would like to offer their experience, please do so in the comments.