Hello!
I've installed Quality Gates Plugin 2.5 and added SonarQube instance in settings.
After that I've created simple freestyle job which has only Quality Gates Post Build with Project Key of one of our project (I've tried different projects, nothing changed)
Than started the job and it worked...
BUT than I want to make changes in my job and every time i'm pressing save or apply button I got error with stack trace starting from:

javax.servlet.ServletException: java.lang.RuntimeException: Failed to serialize hudson.model.Project#publishers for class hudson.model.FreeStyleProject

This only happen if the job was finished at least once.

I've tried to delete the job and got the same error, but on a second attempt it was deleted. I only noticed the difference - after first attempt the job was marked as disabled.

Full stack trace here: http://pastebin.com/cT3SFFMU