If you moved Sitefinity from one server to another you might run into this issue Access denied on App_Data folder.
To solve this issue on IIS 7.5 you need to give permission to the user that is associated with app pool. So let's say the name of your application is xyz.com then you need to go to App_Data folder and go to properties and add IIS AppPool\xyz.com and give read/write access to it.
That should do it.
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