I updated my Win8.1SE with WinBuilder today (20150907), and facing this error in Win8.1SE.
This error appears when I run 'Save/Get Wim Info' button in 'Images Configuration'.
Also, it seems that same error is happening in Win8PESE, too.
Run - Failed to find section [CallC_HostSessionName] in file: [%BaseDir%\Projects\Win8.1SE\Build\Macro_Library.script]
Attached logfile.