Can't save job configuration - JSONObject["hudson-triggers-TimerTrigger"] is not a JSONObject
Since Jenkins 2.264 I am encountering an error when either trying to modify the configuration of a job with a "Build periodically" trigger or when trying to add a "Build periodically" trigger to any job. The error - net.sf.json.JSONException: JSONObject["hudson-triggers-TimerTrigger"] is not a JSONObject - happens when I click on Save or Apply and the job configuration is not saved.
Downgrading to 2.263 resolves the error but that is not a long term solution (especially since I already upgraded some plugins that are now broken because they require a newer Jenkins version ... ).
After 2.264 I tried again with 2.269 today and I still run into this error.
In Jenkins' log I can see the JSON with the job configuration that fails to parse and it contains this: