Problem:
Issue usually occurs in WVD environments where a first user logs in without a problem.
However, any other user after that gets stuck at a windows screen usually showing "Preparing windows"
Resolution:
This is usually caused by a "dirty" FlexApp package .
Specifically one registry needs to be remove from the package.
Open your FlexApp Packaging console and edit the application(s).
Remove the below registry from the package.
MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion and a value of "ProductName"
Product: ProfileUnity
Product Version: 6.8+