MichelDwo Then, the issue is still on Solus's side, and is Desktop Environment independant.
At first I thought I might be able to help, because my workstation runs Solus Plasma, and has two monitors, which perform exactly as they should. My main display (with panel) is on my left monitor, and the extended display is on my right monitor. I chose that arrangement because that's what I used when I was working, but it wasn't the default. That was the other way around, but still no problems at all.
The point is, I had to make that choice in the System Settings. Have you looked there? I also didn't need to install any special drivers. My workstation is powered by a DELL Optiplex, so the graphics are Mesa Intel HD Graphics 630. I simply use the default driver.
So, if it helps at all, it would seem to be a driver issue. Where to go from there is above my pay grade, but best of luck finding a solution. Have you researched what "error -19" represents in this context?