Some users are receiving error 0xA0000400 during 'updating' to Windows 10 Anniversary Update. The upgrade installation terminates. Here are a few background information about the issue and a workaround.
The error 0xA0000400 will be reported from Windows 10 Upgrade Assistant, as shown in the screenshot below. The message doesn't provide any detail.
(Source: MS Answers)
This MS Answers forum thread gives us a clue, that something went wrong with upgrade permission. The thread poster mentioned an embedded industrial Windows 8.1, which perhaps isn't eligible to upgrade. I suppose, the error will be dropped, if a wrong Windows setup images has been downloaded (see upgrade paths here).
And I found several hits searching the internet, that the issue is related to Windows 10 Education (see). As a Microsoft employee mentioned, the issue in Windows 10 Education is known and the developers are working on a solution.
A workaround
If the basis system is eligible for upgrade to Windows 10 Anniversary Update, grab the Media Creation Tool from http://microsoft.com/en-us/software-download/windows10 and download the appropriate Windows 10 install image. Then try to execut setup.exe found on the install media and follow the steps of the update wizard. If that did not work, try to boot the system from install media (DVD, USB stick) and try a clean install. WindowsCentral has a step-by-step guide for instance.
