We have also seen this problem on one of our test servers. The other servers seem to be fine. We have a flow that suspends all of the schedules before we restart in the morning and it has been hanging as described in the first post. But on further investigation, all of the Simple/One-time schedules stopped running hours before the restart was kicked off.
We have noticed that once the Simple/One-time schedules stop running we can no longer get to the Scheduler admin page and the IS will not shutdown if you try and restart it. However any complex schedules that are setup continue to run normally.
At this point, I think I am going to try RMGs idea and recreate the schedules.
Thanks
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