After the scheduler is restarted by changing its configuration, it's not possible to schedule a a task.

Steps to reproduce:

1. Test a common scheduled publication in a page, it works.
2. Go to configuration and modify /modules/scheduler/config/jobs/demo@description with any value so that the scheduler is forced to be restarted. The following traces should appear in the log:

   INFO  info.magnolia.module.ModuleManagerImpl            : Stopping module scheduler
   INFO  info.magnolia.module.scheduler.SchedulerModule    : Waiting up to 30 seconds for scheduled jobs to stop...
   INFO  info.magnolia.module.ModuleManagerImpl            : Starting module scheduler 

1. Try again a scheduled publication in same page than 1, now you will always get the error when pressing Approve & Publish action:

   ERROR magnolia.task.schedule.DefaultTaskSchedulerService: Could not schedule job [publish_9d754a73-bb6e-4e38-97be-9cc4d562e8ed].
   org.quartz.SchedulerException: The Scheduler has been shutdown. 

Although it's not possible to schedule publication tasks, scheduled jobs keep on working fine. It can be tested by setting demo job cron to 0 0/1 * 1/1 * ? * so that it will be executed every minute.

Therefore, the problem is not with the scheduler itself but with info.magnolia.task.schedule.DefaultTaskSchedulerService.schedule(Task) and/or its sub methods.