so little code, so many possibilities!
According to your post, we can rule out references as the problem (or is it just that they are all found but one or more are not compatible with 365?)
A solution might be found in that there's no reason to make the book visible in the first place. Just save it and close it if it doesn't need to be seen.
A potential issue is that for some reason, after the version update the specified window is no longer active and needs to be activated. Not that the Office update would have anything to do with that, just that the update may be irrelevant. Other factors could have changed at the same time, such as coding or how the user interacts with the db.
Or it is no longer number 1 in the collection of windows.
Or don't refer to the application (Excel) by using the ordinal position of a window as a reference. Use the object reference (again, which I don't see due to the minimal code provided).
Or it is a Trusted Locations issue, seeing as how the update could require those locations to be re-established, or so I've read elsewhere.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.