I finally fixed it, too. The Update from Windows 11 22H2 to 23H2 solved it, i guess.
- Download the Microsoft Installation Assistant from here: https://www.microsoft.com/software-download/windows11
- Run the Assistant in Windows 7 compatibility mode as an administrator. Let it search for Windows Updates and accept the upcoming update or if it asks to install Windows 11, accept it (No Data will be lost). The Assistant will upgrade your version from 22H2 to 23H2, but it will take some time.
- After the upgrade the Update KB5034123 should already be successful installed. Check your update history.
- If the Windows Update panel still shows the installation error or download error, suspend the update (choose the option "For a week" or so) and instantly activate it again. Now the system should search for newer updates and install them.