Meticode same thing happen to me, go back and rename the file update (with no .zip).. try it now.
The file is exactly named "update.zip" the .zip being the file extension. Deleting it won't fix my problem because it's finding the file but it's not verifying it.
Did follow the whole tread so forgive me if you did it already, but have you tried re-downloading the file? Maybe it's corrupted.