>>In each of these cases, it's quite possible that it just can't find the file, because the location it runs may not be what you expect - it could be in %windir% or %apdata% and whatnot, and the latter would be under the profile of the user it impersonates - which, again, may not be the one you expect. Many pieces of Windows environment are expected to be found in personalized folders, and when these don't exist, the system just won't tell you the cause of the error. Or it does so in One Microsoft Way, not too useful.
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>Running as a scheduled task.
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>But note that it didn't report "file not found", but rather that the file is invalid. Furthermore, it reported the correct full path to the file. So, I think it is finding the file, but not liking it.

Then see under which account is it running and what rights that account has on the folder.