Having used SSL for a few years now, I've never had a problem on quite a few different setups in different bars/clubs.

Last week I went to a new place, they had two 1200s and a Pioneer 800 mixer (both familiar to me). I hooked everything up, checked a regular record on both tables and it sounded good. After Scratch booted up, I checked the scope screen and saw two very screwed up displays. The left one looked like this:
scratchlive.net
and the right like:
scratchlive.net

Having seen neither of those in all the years of using SSL, I panicked, switched the needles to the respective other turntables, unhooked all the rca cables from the box, etc etc. Suffice to say, I could only get the setup to work in internal mode.

Now, looking through the forum I think it's clear that either both turntables and perhaps the mixer were screwed up and needing servicing somehow, but what I can't understand is how SSL was reading the signal is screwed up, but both turntables played regular vinyl just fine.

Care to shed any light? Thanks.

Scratchlive needs a stereo signal to wrok. Regular vinyl will play on uneven levels. If the left channel is putting out at 20% and the right channel at 100% it will still play, Scratchlive will not.

The left scope view you have indicates that the software is receiving a signal that is missing the left channel. Your right scope view looks like a dusty needle / record.

On the main screen above ABS/REL/INT there is a bar that should fill with grey. On the right side, does it fill with grey or does it have red in there?

Do you have any other needles you can try? You tried replacing the RCA cables, that would have been another suggestion. If you switched your left deck to the right and the right deck to the left, does the scope views follow the deck?

Wow, I'm embarrassed to say I didn't realize channels could be screwed up and regular vinyl can play out, but obviously that makes sense!

This was a one-off gig (I think) and I've used my system on two different turntable/mixer setups and everything is fine, so I'm going to assume it was the house equipment that was screwed up. Thanks for confirming that serato needs a stereo signal and analog phono doesn't! Whatta maroon I am ...

yes, usually those things (scope views) are related to the gear. So if it was only that time on that clubs gear, your Scratchlive set-up is running fine.