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The expansion works though!
I've got the same problem a few minutes ago.

try to refresh your account.

It's ok for me, now.
It's fixed for me too now.
Post edited May 12, 2015 by Teppic
My copy of Towerfall wouldn't start either when downloaded to my Windows 8.1 machine.

In the end I had to download \XNA\4.0\xnafx40_redist.exe from the Microsoft website.
As soon as I did this the game worked fine.

Hope this helps anyone else having issues.