Once set up, never run Windows Update without backing up your DS License Server\bin folder. Keep a copy of your license.xml file. And finally, consider migrating to CATIA V5-6R2022 or 3DEXPERIENCE when your workflow allows – they love Windows 11 natively.
Introduction: The Compatibility Challenge CATIA V5 R21 (released in 2011) is a legendary, stable release still used by many automotive and aerospace engineers. However, it was created long before Windows 11 (released in 2021). Microsoft completely overhauled driver signing, security protocols, and graphics architecture. Consequently, trying to install a decade-old engineering behemoth on a modern OS is not a "double-click and run" affair. Install Catia V5 R21 On Windows 11