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Upgrade to Windows Phone 8.1 Issue:
A number of users have reported issue when upgrading the Windows Phone version. Prior to version 2.5, the app was targeted for Windows Phone 8.0. Starting from version 2.5, it was rebuilt and retargeted for Windows Phone 8.1, incorporating new features not possible on Windows Phone 8.0.
Due to some technical glitch, users have reported either error code c101a7d1 or 80004005. Please refer to below for assistance.
Troubleshooting will need to take place on each specific end-users account and app purchase license. This error code occurs in relationship to the end-users "license" purchase that is generated by the system, not due to a problem with our app, even though it may occur intermittently after a new app update. Most of the users will be able to uninstall and reinstall the issue to resolve this. Just in case there are some who are not able to resolve it that way, you can refer to the following phone support link:
Phone Support: https://gethelp.live.com/en-us/Pages/default.aspx
o Advise to choose "Apps + Store" in the dropdown then click "Contact Us" at the bottom
This will engage the proper support team that can assist further with these error codes for each individual user, as it occurs in direct correlation to the users and their Microsoft Accounts.
They have a few additional support links you can review as well for more information:
Common Windows Store Error Codes: http://support.microsoft.com/kb/2781172
Error Code c101a7d1: