Since the July/August 2026 Windows 11 (Dutch) system update Spotlight (in Windows lock-screen) works again. Note: I did not test the Windows background.
So it wasn't the use of a local account, nor corruption of an account nor several other things and steps in these posts. In short: It wasn't us. It was indeed a bug in Microsoft's programming and they finally fixed it!
Thanks for non-acknowledging this @Microsoft and all the help and suggestions of everyone else here!
Now this bug is finally fixed these feedbacks can be finally closed: https://aka.ms/AA8iflt | https://aka.ms/AAt4ceb | https://aka.ms/AAaslmf | https://aka.ms/AAz1ary | https://aka.ms/AAzt31k | https://aka.ms/AAztamk | https://aka.ms/AAzt31p | https://aka.ms/AAzt31t | https://aka.ms/AAztic7 | https://aka.ms/AAztamr | https://aka.ms/AAzticc