I am using Task Scheduler on Windows 2008 x64. I have 3 tasks, running every night on different times without overlapping. It works for some days - usually 2-3 up to 10 (it's really random), then it stops running the tasks. When I look at the history, I see that the tasks completed successfully. In the UI, the column "Next Run Time" stays empty. The tasks are set to run on background; the account for running them is a domain one - it is valid and enabled. When I check with Process Explorer, there are no left-over processes associated with my tasks. I am completely baffled at what's going on.