Hello,
I've been experiencing a persistent problem with the Preview device for the last few weeks. 
Every time I click on the "Video preview on External monitor" button, I get a black screen on the second screen. When I click into it, the whole Vegas 19 freezes and stops responding after a while.
I have a total of 3 monitors.
Main: 2560x1440 144Hz, secondary: 2x 1920x1080 60Hz.
I have Vegas on the main monitor. And if I set the Video preview to show on one of the FullHD monitors, the same thing happens. Vegas will chop and stop responding.
But if I have Vegas on a FullHD monitor and set "Video preview on External monitor" to show on the main monitor (2560x1440 144Hz), everything works fine. The video will show normally.
The main and one secondary monitor are connected via DisplayPort and one via HDMI port. 
I have tried uninstalling Vegas, reinstalling and the problem persists.
The program has the latest updates, the drivers in the computer are up to date. Even the video card has the latest updates.
My components:
CPU - AMD Ryzen 7 3800X 3.90GHz
RAM - 32GB
GPU - RTX 3060 Ghost OC 12GB
MB - ROG STRIX X570-F GAMING
Vegas version: 19.0 (Build 424)
Where could be the probmlem?
Thank you for the reply