Super Source as Key Source

I am trying to use the Super Source as input to a luma key. I can select it as fill and key source, but it does not work. Is there some limitation that prohibits the use of Super Source als key source?

I am trying to shift the position of subtitles (input is white text on black background) for ME2 without using the DVE (-> flying key).
When I use the external input directly as fill/key source in the USK it works. That means the keyer settings should be correct.

I had the same problem trying to use the Super Source as a key source for a downstream key on the Mini Extreme.