Ok - here are some new Facts on this!!
My current setup is to connect the DJM-900NXS2 with a pretty Long USB cable which got a USB repeater as a standard cable wasn't long enough and couldn't have a stable connection.
To check if this is a USB Connection issue I used my Surface Pro 4 and connected it to the DJM-900NXS2 with the USB cable which came with the unit.
This time the Audio flow via the soundcard of the DJM is stable and fluent.
Therefore I can confirm that for my issue it isn't a Driver nor a rekordbox issue. It is an USB issue.
As I can't move my main PC nearer to the DJM I need to check several other options before. Like using a different USB port, using a powered USB hub or trying another USB repeater to see if this helps.
Any other recommendations? Any suggestions on long USB cables that would work? The lengh I would need is a minimum of 7m.
Thanks
Frank