New Excel workbooks not launching in app - .xlsx "extension is not valid"

Alan_NZ As a seasoned SharePoint administrator, I understand the frustration you must be experiencing with this issue regarding the inability to launch new Excel workbooks from your document libraries. The behavior you're describing, where existing Excel files open successfully but newly created ones return the ".xlsx extension is not valid" error, is certainly perplexing. Let's delve a bit deeper into potential root causes and explore some avenues for resolution.

First, I would recommend thoroughly reviewing the document library settings to ensure there are no unexpected configurations or content types that could be interfering with the creation of new Excel files. Sometimes, even the slightest deviation from the default settings can lead to these kinds of issues. Additionally, I would suggest testing the creation of new Excel workbooks in a separate document library, as you mentioned.

This will help isolate whether the problem is specific to the particular library or if it's a more widespread issue across your SharePoint environment.

If the problem persists in a new library, then we can likely rule out any library-specific settings as the culprit. Another factor to consider is the compatibility between your users' browser and Office client versions, as well as any add-ins or integrations that may be in place.

Ensure that everything is up-to-date and properly configured, as misalignment between these components can sometimes lead to unexpected behavior. If you've exhausted these initial troubleshooting steps and the issue remains unresolved, I would recommend reaching out to Microsoft Support for further assistance.

As a seasoned SharePoint administrator, I understand the importance of maintaining a seamless user experience, and this particular problem seems to be a rather unique one. The Microsoft Support team would be better equipped to delve deeper into the underlying causes and provide more targeted guidance to help you resolve this challenge.

Rest assured, we will get to the bottom of this issue. Please feel free to provide any additional details or context that may help me better understand the specifics of your environment and the steps you've already taken. I'm here to support you in finding a lasting solution.