Win7 x64 Ultimate
I have one desktop PC where this function is not working. I've investigated as far as I can - the scheduled task exists, when it runs I can see CCleaner64.exe spawn in Task Manager, but it then closes after a second and the main window never appears. The status of the task changes from 'Ready' to 'Running', but it gets stuck on 'Running' forever until either I log out or restart the PC. I cannot manually end the task either, it remains in the 'Running' state, event though there aren't any ccleaner processes in Task Manager anymore. If I disable SkipUAC by deleting the task, CCleaner and /AUTO work as expected.
The task action points to "C:\Program Files\CCleaner\CCleaner.exe" $(Arg0). If I change that to CCleaner64.exe, then CCleaner is launched, but still prompts for elevation. When changed back to CCleaner.exe, I get the above described behavior. I've also tested similar tasks to launch other apps elevated, and they work fine.
UAC is enabled to the default level. However there is a slightly different setup on this PC - it uses the default (built-in) Administrator account, which has been renamed, and I manually changed the UAC group policy setting to enable Admin Approval mode:
secpol.msc > Local Policies > Security Options > User Account Control: Use Admin Approval Mode for the built-in Administrator account > Enabled
Let me know if you need any more info to try and reproduce the error.