User can't access Private Teams Channels files

Hello, 

I have a user who is a member of all private team channels (all on separate SharePoint sites). However, they are unable to access files to certain private teams channels. 

They receive this error message every time (see below). Clicking on 'Open in SharePoint' opens a web page and shows Access Denied. 

I've tried the following:

- re-adding the user a member 

- adding the user as an owner

- clearing Teams cache

- reset Teams

- reinstalled Teams

- signed out of account

- using different browsers

- using private/incognito mode

None of these have worked, and I've raised a ticket with Microsoft support, but they have been unhelpful, insisting that it's a permission issue. Even though, I did a remote session with them and showed that the user is a member of all private teams channels. 

Any advice/solutions would be great 🙂