Apple has made several changes in Xcode 9 and in iOS 11.
iOS 11
- Only supports 64 bit applications.
- Requires B4i v4.3+ to develop with iOS 11.
- Make sure that 64 bit is selected under Tools - Build Server - Server Settings.
Xcode 9
- Requires B4i v4.4+. If you are using an old version of B4i then you should install Xcode 8. It can be installed together with Xcode 9.
- The hosted builders are running Xcode 9.