So, after digging around for hours and hours I finally have a solution for this (using UE 4.17.2):
- Remove XCode 9
- Install XCode 8.3.3 (make sure to install it in the applications folder) - download it from Sign In - Apple
- Build!
So, after digging around for hours and hours I finally have a solution for this (using UE 4.17.2):