Problem was using two compilation options at the same time:
"Save a copy of the Scriptsource in the output program resources." on Tab "Resource Update", and ...
"Run Au3Stripper before compilation." on Tab "Au3Stripper" (no options).
In this thread Jos pointed out, that UTF-8 with or without BOM doesn't make any difference.
It was a bug in AutoIt3Wrapper.au3, which was instantly fixed by Jos. That beta Version is available from this page,