VCI-400 No Stereo Sound?

Hey guys,

Some weeks ago I bought a used VCI-400 (I know this subforum is for 400SE, but since hardware are similar, so why not). It works pretty well, but one thing I notice days after purchase is that I can't seem to make any 'stereo' sound coming out from the front headphone jack.

So, there are 3 audio channel option in the playback speaker setting of VCI 400, mono, stereo and quadraphonic; right?
- If it set to mono, the sound works just fine as in mono (when testing the speaker using the test button)
- If it set to stereo, no sound coming out
- If it set to quadraphonic, no sound is coming out from left & right channels, but there are sounds coming from rear left & rear right channels.

Note that this sound problem happens to the whole system, not only inside the DJ software.

Interestingly, when I tried to test the stereo and quadraphonic, there's always a pop-up error that says :
"how to fix The device is being used by another application. if you continue that app may stop working. Do you want to continue? Yes/No"

I've tried many things to solve this 'till now and neither clicking yes, reboot the PC, restart windows audio, reinstalling its drivers from Device Manager, reinstalling the VCI400 driver did the trick, using different headphone cables & adapter. I've tried it on multiple system with Windows 10 and 7.

The fact that there are sound from rear left & right is a hint that the hardware may not be damaged, so it could be that it's just a software error. Well, I hope it is.

Anyone got any idea what's going on?