A custom scheduler entry cannot be edited once the Schedule Start Date/Time is set.
So I setup a new scheduler entry and set Schedule Start Date/Time. After that I could not edit the task the section would not open. I could click on history OK and after clicking on history I could then click on Edit but it only opened History again NOT the edit section.
I got round it by opening the DB table Schedule and changing the entered date/time in column ScheduleStartDate to NULL. After that I could edit the task again.
I hope this works as there's a load of entries required to create a new issue that I do not understand.