Anyone have an answer to this? I'm getting the same error message on a Windows 8.1 64bit machine, running CCleaner v5.03.5128.
Every scheduling option gives this same error, even selecting Run At Logon, where the date selections are greyed out. On a second machine running the same OS and CCleaner versions, the scheduling function works with no problem. Windows Task Scheduler service is running on both machines, so that's not the issue...