This has been broken eight months now.
The status is "NEEDINFO" but all the logs are there and the problem can be easily verified.
I have Xamarin Studio 5.8.3 build 1 on Yosemite and still cannot validate (let alone distribute) apps from Xamarin Studio.
The error I get is:
1. I create an Archive with the correct provisioning profile
2. I click on Validate
3. I am asked username and password for Apple Development
4. I Choose the provisioning profile
5. I receive a Codesign failed error
To validate the app, I have to open the Archive via XCode (6.3.1), where everything goes smoothly.
Why can't this be fixed?