xcode build fails with error code 65 without indicative message

I'm experiencing weird behaviour with my iPhone CI job (running in TeamCity). Every once in a while our build fails without any indicative error in the build log. The build reaches the last stage of codesign validation and I get ** BUILD FAILED ** message immediately after.

These are the last lines of the log of a good and bad builds -

Bad Build
[15:00:56] :             [CodeSign]     /Applications/http://ift.tt/1dP4fCn /Library/TeamCity/buildAgent/work/dc055fa257f562be/DerivedData/MyProject/Build/Products/AdHoc-iphoneos/MyProject.app
[15:00:58]W:     [Step 3/6] ** BUILD FAILED **

Good Build
[14:09:13] :             [CodeSign]     /Applications/http://ift.tt/1dP4fCn /Library/TeamCity/buildAgent/work/dc055fa257f562be/DerivedData/MyProject/Build/Products/AdHoc-iphoneos/MyProject.app
[14:09:13] :     [Step 3/6] ** BUILD SUCCEEDED **

Because it's running in a CI I looked for user interaction problems (opening the keychain...etc') but usually xcodebuild spews the relevant errors.

Is there any other log I can dig into to find the cause of the build failures?