Hello!
Thank you for posting in Microsoft Community. We appreciate you share with us all the methods you have tried.
The brown box icon on the left corner of the documents in OneDrive for business indicates that the file may be corrupted or damaged. This could be due to various reasons such as file synchronization errors, or issues with the file itself.
You may go to OneDrive Online in browser and see if you can open those documents. If yes, try downloading it from there and see if it opens.
If the documents are not available in OneDrive Online, you may check if they are in OneDrive recycle bin.
If the above steps do not work or you can't open those documents from OneDrive Online, I understand how frustrating this situation can be and sincerely apologize for the inconvenience caused.
I'm afraid the best way to troubleshoot the issue further is to let support team who has higher permission to check those documents from back end. So, I suggest you raise a support ticket from the Microsoft 365 admin center, where the related team will collect the back end logs and find out the cause of the problem. This would be the most efficient way in handling this case for you.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have Microsoft 365 admin privileges.
Apologies for redirecting you to the related team support as the moderators in this community have limited resources to check the backend information, and to get the fast and better assistance we requested for it.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Tina | Microsoft Community Moderator