ElasticBox plugin is using github plugin to connect to GitHub. It uses Crendential class that GitHub-plugin has removed in 1.13 release.
It seems that there is a migration to move the class from com.cloudbees.jenkins.Credential.java to org.jenkinsci.plugins.github.deprecated.Credential.java but when we install ElasticBox plugin, that migration is failing. I've attached jenkins log.
It seems also that you removed login() function from Credential class, so although migration works, the compatibility with a plugin that is depending on this, is not working anymore.