An updated version of the Assessment and Deployment Kit (ADK) for Windows 8.1 was released last week. Details can be found in this blog post:
If you have System Center 2012 R2 Configuration Manager or if you are following the instructions in this post and you downloaded and installed the ADK 8.1 before 10/18/2013, you should download and install this updated version as it contains fixes for a few issues. You do not have to uninstall the previous version. If you simply run the ADK setup it will update the previous install. You also need to update the User State Migration Tool (USMT) package created by the Configuration Manager install as this update contains fixes to the USMT binaries.
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