https://answers.microsoft.com/en-us/windows/forum/all/update...

https://answers.microsoft.com/en-us/windows/forum/all/update...

https://answers.microsoft.com/en-us/windows/forum/all/window...

https://answers.microsoft.com/en-us/windows/forum/all/window...

and numerous others.

Why does it happen? I have no idea, and probably Microsoft doesn't either. Nevertheless, it means the update process is broken. After such a case you may feel like you have the alternative of either running unsecure system or losing your data, so you may even consider the first option. Except you can't unless you pay for the Enterprise version.