I haven’t met the error you are having, but sometimes the --deep command doesn’t sign everything and you have to sign everything manually. We’ve written an automated workflow / guide to prepare distribution for OSX (from start to finish) in and outside of the Appstore together with an extensive guide I would suggest reading it. The code to do it manually is also included and I would suggest you read up at signingpackage if you scroll up a bit it shows what other packages to sign. Good luck!
You can read the full text guide with DIY here and get the full here.
You can find the Unity forum thread here .