Task sheduler over GPO doesn't allow domain admins to save and run jobs, this is major problem.
Microsoft did a big crap when they cancel that. Instead of helping us, they make problems. Because with user NT-system, nothing works!
So you need to create job on client as administrator and than robocopy will work.
Second less safe action is to create jobs on one client and than save it and than make a batch which you run it over GPO to all clients. This way robocopy will start as administrator and it will work.
I never left an open problem....I search, dig and ask, until it's solved....