This error can occur if you have a new user that took the same email address/user ID as the previous user. Per this article at Microsoft, here's how to fix it via Powershell. We ran the command against the main Sharepoint site (top) and also the user's OneDrive who was sharing out the documents. This removes the previous user's user-info entry.

Remove-SPOUser -Site https://your-365-tenantid.sharepoint.com -LoginName user-email-who-cannot-access-attachments@your-365-domain
Remove-SPOUser -Site https://your-365-tenantid-my.sharepoint.com/personal/user_name_whos_sharing_yourdomain -LoginName user-email-who-cannot-access-attachments@your-365-domain

Remove-SPOUser -Site https://your-365-domain-my.sharepoint.com/personal/christina_motipt_com -LoginName user-email-who-cannot-access-attachments@your-365-domain