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Dear Rick,
Good day! Thank you for posting your query in our community.
Per your description, we guess the process is that a user in company shares fils with an external user through a link, when this external user logs in to the invited account to access the shared file, the error occurs.
If my guess is correct, please first make sure this external user have been added as guest user in the company.
If this external account has been added as guest user in the company and the problem persists, please delete this guest user in the Microsoft 365 admin center or Azure AD, and follow the steps in this part Remove users from SharePoint - SharePoint in Microsoft 365 | Microsoft Learn to wipe the user out of the site collection completely (from the user information list), then re-assign the permission to the user to see the result.
If the problem persists, could you please provide more information for me?
- You can switch to different browsers/inprivate window to see if the same problem happen.
- Does the problem happen when another user in the company shares the file with the same external user?
Sincerely,
Jazlyn | Microsoft Community Moderator