Task scheduler repeat popup "task scheduler service is not available. Verify the service is running."

Recently I noticed a few different errors on task scheduler on some machines that had gone through in place operating system upgrades from Windows 2008 to Windows 2016. After the upgrade, when opening the task scheduler gui tool, it either gave popup messages repeatedly saying the service was not available despite the service being started. Clicking through many of these, it would eventually show some tasks in the list, but not all. Also schtasks.exe was working fine and showing all the tasks. On some of the machines, the gui tool was just giving snap-in errors and nothing would load in the tool. 

The issue turned out to be from some of the legacy jobs. In older versions of windows, simple scheduled tasks could be created through the at.exe command. If some jobs existed on the machine prior to the upgrade, they would still be in the windows directory, and will still show up in schtasks.exe. You can't edit or delete them with the schtasks command line tool, and they won't show up at all in the GUI after you click through the errors. If you try to use at.exe command it says its no longer supported and won't run. You can just delete the job files from the c:\windows\tasks folder. After that, the errors in the gui tool will go away.