Did you try deleting the maintenance plan or try removing the schedule from the maintenance plan?

I think you are probably hitting an issue with the maintenance plan, a bit of a known issue.

If you still get the errors trying to remove the job from the maintenance plan or trying to delete the maintenance plan, you need to delete the log (from sysmaintplan_log) and subplan (from sysmaintplan_subplans) for the maintenance plan.

Sue