After upgrading jenkins and plugins, I installed scm-sync-configuration plugin, but did not configured it: I chose "None" as SCM in the configuration page.

Any changes to a job configuration hang after that. Looking in the log I see errors like this:
Dec 08, 2015 5:42:37 PM hudson.plugins.scm_sync_configuration.ScmSyncConfigurationBusiness queueChangeSet
INFO: Queue of changeset A hudson.plugins.emailext.ExtendedEmailPublisher.xml
A hudson.scm.SubversionSCM.xml
A hudson.maven.MavenModuleSet.xml
A hudson.tasks.Ant.xml
A hudson.plugins.git.GitSCM.xml
A hudson.triggers.SCMTrigger.xml
A hudson.plugins.git.GitTool.xml
A scm-sync-configuration.xml
A hudson.tasks.Mailer.xml
A config.xml
A hudson.tasks.Maven.xml
A hudson.tasks.Shell.xml
A hudson.scm.CVSSCM.xml
aborted (scm manipulator not settled !)

I remain unable to save job changes and because I have other long running jobs, I can't restart jenkins either until these jobs complete (which will take days)

If the plugin is NOT configured, it shouldn't attempt anything.

Laura.