Cannot Update Version

I am trying to update the premium version from 1.4.9 to 1.5.1, but I am getting the following error: Update Failed: The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.

I believe all the permissions are set up correctly, 775 on the folders, and 664 on the files. Any idea how I can get this to install? Thank you.

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