Can't save job configuration - JSONObject["hudson-triggers-TimerTrigger"] is not a JSONObject

[Inga Quatuor]

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:

"hudson-triggers-TimerTrigger": true,
"spec": "H 1 * * *",

based on the exception I suspect that this should be

"hudson-triggers-TimerTrigger": {
        "spec": "H 1 * * *"
 }

instead but I have no idea how I can fix this at my end.

If anyone knows a solution or a workaround I'd greatly appreciate it. I would report this at Jenkins Jira, but I can't login there :/

Thanks, Inga