iOS XCode code sign fails

Hi,
I know this is a common question but I have troubels building my app in XCode. It works fine on my MBP but when I switch to my iMac, I get the error

Code Sign error: The identity 'My Name' doesn't match any valid certificate/private key pair in the default keychain

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I did import my dev key/cert pair into key chain
I do have a provisioning profile installed
and I did select my key in the code signing options in xCode

What did I forget ?

jnoxx said:

Apple Worldwide Developer Relations Certification Authority

Do u have that certificate too?

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I haven't explicitly imported the Apple's CA certificate but Key Chain shows my developer cert as valid the certification chain is verified.

Edit: I imported it from here but I'm still getting the error

Edit2: if I open the project.pbxproj file, I see that there are still some references to my old signing profile ?! If I replace the

"CODE_SIGN_IDENTITY = MyName";

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by

CODE_SIGN_IDENTITY = "iPhone Developer: MyName (XXXXXXXX6)";

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then it works. I don't understand why xcode was not showing this old signing identity ...