I have not seen any other report of a similar issue so it could be my environment. I am reluctant to delete my Jenkins installation and start from scratch since I have many jobs. I suspect I have a bad plugin, but again I wouldn't know which one to disable.
I am unable to modify jobs other than hand-editing the xml file, which is a major issue for me, but since no-one else has reported a similar problem, it can't be an issue for everyone.
I very rarely modify jobs so I don't know when this problem started happening