Here's my response from Microsoft on X.
Thanks for sharing the additional information with us.
After reviewing this case, we found that it’s an ongoing issue, and our engineers are actively working to resolve it. Although we don’t have a specific timeline for the fix yet, an internal ticket has already been raised, and we’re optimistic that a solution will be available soon.
To help us move closer to resolving the issue, please submit a detailed failure report from within Excel. This will send diagnostic logs directly to our product engineering team. You can do this by going to Help > Feedback > Report a Problem, and preferably using the following text: "Stock Data Types failing to refresh with server error immediately after 2026 start. Impacting all users."
Once you’ve submitted your feedback, we recommend monitoring the situation and keeping us updated on any changes.
Your patience and understanding are greatly appreciated. If you need further assistance, we’re here to help and will be more than happy to support you. ~AD