OS X 10.10 gatekeeper gives 'app damaged put it into trash' message on 10.10 but everything's ok on 10.11

Hello,

On OS X 10.11.4, I compile and codesign my application. I am using QT 5.5 and macdeploy, in addition to codesign.

Then, I verify using both:

codesign --verify --verbose myApp.app

spctl --assess --type execute myApp

before I create the myApp.dmg disk image.

Everything is ok on OS X 10.11.4, when I open the disk image and copy myApp.app on the mac.

However, on a mac running 10.10.5, when I try to open myApp.app, I get the following error:

"myApp.app" is damaged and can't be opened. You should move it to the Trash.

I checked with the "RB App Checker Lite" and I don't see any error.

I had a previous version done in November 2015 which works perfectly on both OS X 10.10 and 10.11.

Am I missing something basic?

Thanks for any help