Microsoft has shared details of a new known issue with Windows 11. The company has confirmed that a problem exists with apps that use certain characters in registry keys. From a report:
As a result of the discovery, Microsoft has put a compatibility hold in place that means people with problematic apps installed will not be offered Windows 11 via Windows Update. The issue is under investigation. It seems that the issue is related to, or is an extension of, one of the three initial known issues with Windows 11.