Hello Tim G,
I’m Segunfunmi, an Independent Advisor and Microsoft user like you. Thanks for posting the query here in this forum.
I understand your issue with SharePoint's "Access Denied" problem. Here are potential solutions:
Check User Access Diagnostic: Microsoft 365 administrators can run diagnostics within the tenant to identify issues affecting user access.
Manual Verification: Verify permissions using the "Check Permissions" feature. For SharePoint, go to Settings > Site permissions; for OneDrive, go to Settings > Site Settings > Site permissions. Then, select "Check Permissions," enter the user's name in the User/Group field, and choose "Check Now."
User ID Mismatch: An ID mismatch can occur when a user account is deleted from the Microsoft 365 admin centre, and a new version is created using the same user principal name (UPN) used by the deleted account.
Throttling: If users receive "503" or "Server is busy" error messages, these issues may be caused by throttling within the SharePoint service.
Reference:
https://learn.microsoft.com/sharepoint/troubleshoot/administration/sharepoint-online-inaccessible
Meanwhile, if you still need further help or I misunderstood your question, to better help you, please kindly correct me and share more details with me so that I can do further research. Thanks.
Thank you in advance for your understanding! Your patience and cooperation will be greatly appreciated. I wish you all the best!
Kind regards, Segunfunmi.