Schedule (defrag a drive) Error

When trying to set up a scheduled defrag I get the following error:

Defraggler was unable to change the options

You are not the only one.

Normally, Defraggler works good when setting a schedule on a PC.

But sometimes it happens that on certain machines, it fails with that exact error.

This is independent of OS, as I had it happen on 7, Vista, & XP.

I am not sure what causes it, but perhaps someone else here would like to give an explanation?