Hi, I cannot be 100% sure, you really did not provide much information, but I believe your solution is not using the IsolatedStorage, you see your issue ("Initialization failed") occurs due to it.
Unfortunately you don't have a control over this, the OpenXML SDK uses System.IO.Packaging namespace which behaves in a way that when manipulating with the file that is bigger than some threshold size then it will begin to use IsolatedStorage using a user/domain store as a scratch pad area. To access IsolatedStorage your code must have the necessary privileges otherwise it will fail.
Also you should read the following Eric White post:
http://openxmldeveloper.org/discussions/formats/f/14/p/7288/162499.aspx#162537[
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