Engine Connect failed connections to computer via Denon X1850 mixer and SC6000M's

I hope you put in a support ticket.

Meanwhile, beware, the X1850 got changed along the way and per this thread the 1.6 firmware doesn’t work on all models.

see @DJJV 's comment at post 40

Older firmware is posted, but 1.6 is not, presumably for that reason.