Jumping in late.
Were you taking your headphone out of your laptop to your mixer?
What did you get, a matching transformer to make it work?
Personally, I'd have use a direct box, either 2 mono's or a stereo, and preferably active. That should ensure a proper impedence matching. Then again, from the live sound world, we want it all to look like a microphone at the input!
No doubt it was a combination of RF bleed and AC/DC noise creeping into the signal
Not to mention not ADJ brands, but get Furman products to protect your power(and gear!). It helps a LOT with this sort of stuff. I was doing a gig on an air force base(one of 3 I had done there) and going from house direct to anything was a bad thing. Put in the Furman to filter my power and all the RF flying around was nuked and the power was cleaned up.
But if you're going out from an USB device presenting line out(probably -10) to your mixer, you SHOULD be fine, but obviously there's still some electrical issues. Stay on the same power strip with the mixer for the laptop. It helps a lot.