The following are 5 useful sites for SharePoint Online Troubleshooting:
SharePoint Online troubleshooting:
https://docs.microsoft.com/en-us/sharepoint/troubleshoot/sharepoint-online-welcome
How to troubleshoot issues in SharePoint Online and OneDrive using diagnostics:
Office 365 SharePoint Online performance troubleshooter:
Diagnosing performance issues with SharePoint Online:
Troubleshoot issues with SharePoint: