Since the DiRT code is identical when you're running scheduled vs. running interactive (the only difference is it launches automatically and will not put dialogs up - all the backup code is the same), it's not an issue with DiRT's logic somehow being flawed for scheduled backups. The same lines of code are being executed, so if behavior changes when running scheduled vs. interactively, you need to look externally for a reason.
I'm not sure I understand what you mean by "local share" and you seem to contradict yourself here - you say it works when you do it manually but then you say you get an error when you try and manually copy the local backup to the network drive. When you run it manually I assume you are doing the same two hop backup that you have scheduled, right? I have no idea what you are seeing with a "path is too deep" error there - sounds like some other problem with the way your doing the manual copy.
Have you tried using a mapped drive instead of UNC out of curiosity.
Assuming you have the scheduled task setup to run under the same account you are testing with at the desktop and all other variables are the same, I'd look at something like CSA or a virus package shutting it down when it tries to write out to a remote location.
