When in Teams, if I click the Files area (under Calls), I see "404 FILE NOT FOUND".
This is the case for the webapp, and also the desktop app. All users are seeing this.
File are still available in the channels, OneDrive and Sharepoint are working, this just affects the main File area.
The only change was that I added a custom domain and set the domain of the users (therefor their login) to be at that domain rather than the onmicrosoft one.
Not seeing any other issues in any other areas. I had a hunt around, and can't see a setting where this can be pointed somewhere, and the other people with the 404 error are related to specific documents.