Out 1/2 are the same output; 3/4 would be CH2, per the second image you posted.
What I got from time to time (right now I cannot replicate the issue if I want to) is that I get a low volume signal on channel 2 of the mixer when USB input is selected. Main singal is on Channel 1 of the mixer - which is fine.
Setup is that Channel 1 is selected for USB input (doesn't matter if USB 1 or 2 is used, as long as Channel 1 and 2 has the same selection) and rekordbox DJ is set up as Output 'Out 1 and Out 2'


When the issue occurs I got output on the mixer on Channel 1 - as usual with the correct volume and on Channel 2 (which should not happen) with a low volume.
When powering down the Mixer and power it on again, it is back to normal (no sound on channel 2).
I must admit that I haven't seen it since I changed to the new Windows 10 driver (v 1.200). I will keep an eye on it, but wanted to check if someone else experienced this?
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Out 1/2 are the same output; 3/4 would be CH2, per the second image you posted.
I know. Maybe I explained not clear enough. CH2 on the Mixer got the same Sound in Addition to CH1. According to rekordbox Setup it should be CH1 only.
As I don‘t configured 3/4 in rekordbox, CH2 should be silent.
Let us know if it happens again -- as you state, it hasn't happened since updating.
@Pulse.
Sure, will keep an eye on it.
btw. When I logged in via my iPad through Safari this time, it worked just right away. 😀