Hi Martin,
This can happen due to a bad download. Please use the ISO and let me know if that helps.
- Download Windows 11 Media Creation Tool (MCT) from this link:
https://go.microsoft.com/fwlink/?linkid=2156295
- Run MCT and download the ISO it recommends for your system. Do *not* uncheck "Use the recommended options for this PC".
- Right-click on the downloaded ISO and choose "Mount." This assigns the ISO a drive letter.
- Run setup.exe from the ISO drive. This starts the in-place upgrade/repair installation.
(Your files, settings and applications will be preserved if you use the ISO recommended for your system by the Media Creation Tool.)
Additional note:
To ensure a smooth upgrade experience, follow these steps before beginning the upgrade.
- Enable Memory Integrity. To enable memory integrity in Windows, go to Windows Security > Device security > Core isolation details > Memory integrity. If it shows any incompatible drivers, post the screenshot here.
- Perform a clean boot on the system before running the setup to avoid third-party interference. A clean boot is the process of starting Windows without loading any third-party applications or services. For more information, please refer to the Microsoft article.
How to perform a clean boot in Windows - Microsoft Support:
https://support.microsoft.com/en-us/topic/how-to-perform-a-clean-boot-in-windows-da2f9573-6eec-00ad-2f8a-a97a1807f3dd
(Step 3 in the above link is very important. You need to first enable "Hide all Microsoft Services" before clicking "Disable all".)
- Uninstall 3rd party antivirus if installed; you can reinstall it after the upgrade. After uninstalling it, restart Windows.