After the installation of Jenkins, I installed, set up the xcode plugin and created a few jobs. They ran just fine but now, after a restart each build cancels with the error message:

"Code Sign error: The identity 'iPhone Distribution' doesn't match any valid, non-expired certificate/private key pair in the default keychain"

It looks like an issue with an unlocked keychain, but if I open up a terminal, log in as the 'jenkins' user and perform the commands manually:

$ su jenkins
$ /usr/bin/security list-keychains -s ${HOME}/Library/Keychains/jenkins.keychain
$ /usr/bin/security default-keychain -d user -s ${HOME}/Library/Keychains/jenkins.keychain
$ /usr/bin/security unlock-keychain -p ******** ${HOME}/Library/Keychains/jenkins.keychain
$ cd /Users/Shared/Jenkins/Home/jobs/Puzzle/workspace/Puzzle/
$ /usr/bin/xcodebuild -alltargets -configuration Release clean build CONFIGURATION_BUILD_DIR=/Users/Shared/Jenkins/Home/jobs/Puzzle/workspace/build

...the build finishes successfully. (for the terminal calls, I replaced ${HOME} with my installation path /Users/Shared/Jenkins/Home/)