Dear Craig la Buscagne,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern but since you have tried clearing the cache with no success, I would like to confirm whether you have enabled the custom script on the problematic SharePoint Online site: Allow or prevent custom script - SharePoint in Microsoft 365 | Microsoft Learn
If you have enabled the custom script on the problematic site but the problem is happening, then I would like to suggest you check whether SharePoint Designer is enabled at your site:
Open the problematic SharePoint Online site> Settings> Site Information> View all settings> SharePoint Designer Settings> Make sure Enable SharePoint Designer is enabled.
If it’s already enabled but still the same problem happens, then please try the following steps and check the outcome:
Step 1: Press Windows logo +R > Enter regedit> Click OK
Step 2: Go to HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Common\Internet\WebServiceCache\AllUsers\office15client.microsoft.com
Step3: Delete all the subfolders in the office15client.microsoft.com folder.
WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Edit at your own risk.
If still the same issue happens, we are sorry and I believe the problem should be checked with the related development team, where they can investigate the problem from their end via collecting the logs by reproducing the problem.
Note: I have checked the service health dashboard at the Microsoft end, where there is no related incident found.
So, I request you to connect with your IT admin of your organization (How do I find my Microsoft 365 admin? - Microsoft Support) and raise a new service request from the Microsoft 365 admin center so that the related development team will connect with you and investigate the issue further.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: We apologize for redirecting you to related development support team because the moderators in this community are limited with the resource for further investigation and to get a quick resolution to your problem, we requested it.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia
