For that price you can get a new interface.  My suggestion either an Audient id4 or id44.  Better quality than the Focusrite, I'd bet (the pres are from their consoles) and good to great conversion.  Along everything else you might need, at least in the 44.  Even if the quality gains be smallish, you'll have a brand new board.

Another stupid question. Have you checked the software app to make sure the balance hasn't been tampered with. Doesn't sound like a main went bad because you have both sides on some channels....and you do know some channels are mono right?

That is a good catch to test. MixControl has a setting to restore factory defaults in the file menu IIRC.

RME has a total-reset button in Total Mix that has saved me any number of times.

I think Scarlett 18i20 Gen 1 has to work with MixControl. So if there is a setting in MixControl to do a factory reset I would certainly be trying this.
 
Focusrite Control is the current software for all their newer interfaces. From what I can see there is no such factory reset under Focusrite Control. 
 
Starise brings up a good point though. The other day I was hearing everything in just one ear under Focusrite Control (with a Clarett 2 Pre interface) and it was just a silly balance control being out of whack. I did discover something unusual the other day though doing a stereo recording through a Clarett 2 Pre interface. If you monitor with headphones plugged into the Clarett 2 Pre at a very low volume the headphone mix automatically goes into mono! The stereo does not appear until you turn the phones up to a higher level. That one threw me!
 
It is rare (but not impossible) for a hardware fault to occur though. This sounds like it might be just that. An output chip has failed etc... 

Aw YES!  More very good suggestions and ideas.  Two heads are truly better than one.  I have performed the factory reset several times. 
 
I have not given up just yet.  I have done that at least once in the past and regretted it later, so I will follow all the suggestions above and be meticulous in trouble shooting.  I have been through the scenarios more than once, but maybe the third time will be charm.
 
It might be a while before I get to it.  I am recently located in another town because of work so I don't get "home" often, but as soon as I do I will get at it.  In the meantime, just for fun, I have started to look at other units.  The Audient suggestion was interesting because I have never heard of them.  Some people report latency issues with those products, so I continue to research.
 
Thanks again for all the help.  I will llet you know as soon as I get a chance to try again.

Jesse, is it an output that has failed or an input?
 
In MixControl, verify that any internal routing has not been altered.  Basically, anything in MixControl's middle section is how the unit can be internally matrixed, separate from DAW routing.
 
All in all, I suppose it's possible that a "channel has failed," but seems odd. I'd suspect a loose connection over a failed DAC or ADC.

Here's a final verify test suggestion. Hook it up temporarily to a different computer and install the drivers. Then just patch any audio source the computer may have through it. Even just media player should work. If you install it fresh on another machine and you still have no output on the right side, then its likely a hardware problem and not worth pursuing further.

JohanSebatianGremlin
Here's a final verify test suggestion. Hook it up temporarily to a different computer and install the drivers. Then just patch any audio source the computer may have through it. Even just media player should work. If you install it fresh on another machine and you still have no output on the right side, then its likely a hardware problem and not worth pursuing further.

Yes JSG, this I have done. I had the same results on the second computer. But I will still give it one more go round because I want to confirm my confirmation
 
It is hard for me to give up this unit, so I have a touch of denial, but all things must pass.

If it were a higher-end audio interface (upwards of $2k), it would be worth having repaired.
 
Sub $500 interface that's 5+ years old, you can replace it with a newer model for same cost as repair.
No down-time... and you'd have slightly better A/D D/A... and most likely lower round-trip latency (depending on the model).