Odd results with loopback test

[unicast]

Hi! I've been trying to google my problem, but it seems either very unique or I don't understand some very simple basics of REW.
I'm installed REW 5.20 on Mac OS 10.16 and trying to run a simple loopback measurement of my Audient iD14 MK1 (out 1 goes to input 1).
I adjusted the levels to match, but when I do a measurement I'm always getting a picture like that:

I can't believe it's true because such distortions will be very audible.
I also tried to use two other ports in different combinations, using different levels and adjusting both in and out buffers - same results.
Any clues?

 

[John Mulcahy]

There is some monitoring active, so some of the input signal is being fed to the output. Monitoring must be completely off.

 

[unicast]

Yes, that was one of my first clues and I made sure that the ID software (that turns on live monitoring) is switched off and tested with another audio source that no sound is coming through.

 

[unicast]

Ok, thanks for your suggestion, I tried one more test with ID Software running and both of the input channels muted - now it's showing me a flat-ish curve. The issue seems resolved, thank you for a very prompt answer!