Suddenly we are unable to open xlsb Excel workbooks with Excel complaining "Excel cannot open the file because the file format or file extension is not valid." The files are on a network drive but the issue happens if they are copied to a cloud location. The error occurs when opening the files directly from our laptops but if we use a VM (virtual machine) the files open fine and work normally. The files are xlsb with macros and custom ribbons. However at least one is xlsm with no ribbon.
There were some security patches rolled out from Microsoft which I suspect have broken something. Anyone else have this issue? reminds of a very similar event about a decade ago.
It was caused by a change in internal security. The team were configuring Attack Surface Reduction rules for test/audit but they accidentally got enabled/enforced.