As I understand it, if you create .icns in Icon Composer, this will lead to a verification failure, even if the correct 512x512@2x present. Use iconutil to convert .icns back to an icon set, and then use the icon to create a new .icns from the icon set.
For instance:
resources $ iconutil -c iconset icon.icns resources $ iconutil -c icns icon.iconset
Additional information on iconutil is here .
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