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

After upgrading Jenkins to 2.264 or higher I am unable to save any job configuration that uses a "Build periodically" trigger. If I modify an existing job with such a trigger or create a new job and include such a trigger and try to save, the following exception shows up and the configuration is not saved:

JSONObject["hudson-triggers-TimerTrigger"] is not a JSONObject.

Stack trace and Jenkins log for "doConfigSubmit" attached.

Steps to reproduce:

• Create New Item -> Freestyle project
• Select checkbox "Build periodically" in section Build Triggers
• Enter any schedule, e. g. "@mignight" or "H 1 * * *"
• Click save -> ERROR

As far as I know both freestyle project and periodic trigger are part of core Jenkins and not of any plugin, but I am not completely sure about that. If I should provide a list of installed plugins or check for any specific one, please let me know.

I reproduced this bug with Jenkins 2.264, 2.269 and the latest version 2.274. Downgrading to 2.263 resolves it.