If I set the transparency with WinSetTrans, screenshot can't be made.
This is Windows 7 related.
Win 10 works fine
Run the attached file
#include <ScreenCapture.au3>
;GUI1
$gui1 = GUICreate("gui1", 200, 200, 200, 200)
GUICtrlCreateLabel("GUI 1 LABEL", 50, 50 )
GUISetState()
;GUI...