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.