The scheduled job is by default run as an anonymous user, while it will run in your current user's context when you trigger it manually. Also your exception message seems to be a custom one so that should give you clues where in your code to look.

Guess you have references to HttpContext.Current inside your method. That will not work... cause of reasons Magnus explained

But then it should fail when running scheduled in the local environment as well. But with the anonymous vs permissions problem there could be a difference in access rights between the two environments.

Aha, helps to read all the words in the orginal post,, you are right magnus

Is the scheduled task writing to disk? You need to give IIS_IUSRS write access to the directory you're writing the sitemap too (if this is the case).

/Steve