I have same problem with DIrectX "INVALID_ARG" with my 21:9 LG UltraWide
First step was -
Originally Posted by The Composer
If you're referring to the Google drive folder Raze linked, I uploaded a mirror for you here (https://autodidactstudios.com/Downloads/DOS2DE_Doc_36326602.zip).

"After that, extract the replacement folder from this download into your '..\Documents\Larian Studios' folder, and see if that will let you start the game."

After that I was able to start game in windowed mode and play. But Fullscree or Windowed without frame still was lagged.

And now i finally understand what was problem. I used the wrong cable
PC (DisplayPort) - Monitor (HDMI) - not work properly
PC (HDMI) - Monitor (HDMI) - worked fine.

May be was helpful for someone.

Thnx!