Have you tried doing a little bit then coming home, then a little bit more, etc? My oldest female dog(non GSD) will not go outside if a coyote has been out there. I could always tell when a coyote was in the yard..for a couple days after that I had to take her out through the front. So maybe your dog is not afraid of something it seen, but rather something it smells or senses?
Misty- Samoyed Mix
Tannor- Golden Retriever CGC
Robyn- German Shepherd CGC
Cats-Thunder, Harley, Miley, Bandit, and Ferah
RIP Boo..Black Lab
"A dog is the only thing on this earth that loves you
more than he loves himself."