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Dear webbrewers,
Good day! Thank you for posting in Microsoft Community. We are happy to help you.
As per your description, it seems that you are trying to use SP designer to customize SharePoint site, however, error prompt, and you don't want to change registry in troubleshooting.
Based on our research on the error, there are multiple users who encounter the same error resolve the issue by modifying the ADAL registry.
Personally, I strongly agree with you that we don't want to take risk modifying registry to troubleshoot the issue that's caused by the others. We do understand the inconvenience caused and apologize for it.
Given this situation, since we don't have the resources to check what changes might have happened from backend log. We'd recommend you contact your office 365 administrator to go to Office 365 Admin Center-> Support-> New Service Request. The engineers in related team have higher permission and resources than us to help check the root cause from background directly. The support engineers there have the correct escalation channel, and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket. Ways to contact support for business products - Admin help.
Additionally, here are some threads of similar issue for your reference, hope the resolution steps mentioned in these threads can help:
Error opening site in Sharepoint Designer "Your account is in a - Microsoft Community
Your account is in bad state.Please sign-in to this account online to - Microsoft Community
SharePoint Designer 2013 login Issues in a cloud scenario - SharePoint | Microsoft Learn
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Rhoda | Microsoft Community Moderator