Hi @underwood
Based on my knowledge, MSAL API enforce accounts to be authenticated with Azure AD. If the old account has not been migrated to Azure AD, it may cause access failure.
Older OneDrive accounts are more likely to be supported Credentials API, as it may leverage pre-existing legacy authentication mechanisms or cached credentials.
As your issue is related to API. It is recommended that you post the thread to OneDrive for Developer forum for more comprehensive information:
https://techcommunity.microsoft.com/category/onedriveforbusiness/discussions/onedrivedeveloper
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us.
Thank you for your understanding.
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