Hi Hazelnut. Yes, just one user on both, which are Local Account, Administrator accounts. However, I think you hit the nail on the head.
The task I exported from my other computer still had my other user name in it. So, I changed the user name in the XML and re-import it, although it still didn't work. But, looking at the other tasks in task scheduler, I think the formatting for the username has changed and it now needs a computer name too. After I changed the <UserId> from [username] to [Computername]\[username], it imported OK & works fine.
UserIdformat
[Computername]\[username]
Old style:
<UserId>user</UserId>
New style:
<UserId>computer\user</UserId>
It's not totally perfect though, by exporting the XML file and renaming the user ID, CCleaner works without UAC prompt. But, if I go to Options > Advanced > Skip User Account Control Warning and untick the check box, when I try ticking it again, the problem returns and it deletes the task from Task Scheduler.