Offizieller Kommentar
Hey guys,
The problem is that you're using software that doesn't necessarily recognize the DJM-450's drivers because the application doesn't support ASIO. Instead, it's pulling the WDM or WASAPI drivers, which don't allow more than the first pair of inputs / outputs to be used by the application. By default, without rekordbox running, the recording output is set to outputs 5/6, which means that anything NOT using ASIO will be unable to use those outputs.
I just tested this using rekordbox, Virtual DJ, Traktor, and Ableton Live, all on Windows 10, and was able to record without issue. Audacity does NOT support ASIO out of the box, but can be configured to support ASIO devices by compiling it yourself, but will not be able to record from them otherwise.