Dear jleonq,
Good day!
Thank you for posting to Microsoft Community. We are glad to assist!
Based on your description regarding "Why can't I sign in to my Developer Program dashboard?" Kindly check this article about Microsoft 365 Developer Program FAQ | Microsoft Learn if it can provide any suitable information about your mentioned concern. In the meantime, I would like to share some information with you, since we have dedicated support team for Developer program related query. So, we would recommend you to kindly post your query to Visual studio support Forum via Visual Studio Subscription Support - Visual Studio (microsoft.com) Engineers who are dedicated into this environment with rich experience and more resources can then assist efficiently. We believe they would conduct in-depth research on the problem you encountered and provide you with more professional assistance.

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In the meantime, I would also like to invite community members and MVPs in the forum to share their knowledge or any workaround and insight on this topic.
Thanks for your precious time and your understanding would be highly appreciated. Have a great 2024!
Best regards,
Inema | Microsoft Community Moderator