Why doesn't macos output to an external monitor?

I

italant2021-06-22 21:24:23

macOS

italant, 2021-06-22 21:24:23

I am trying to connect an external monitor Lg 27UK650 to imac 2017 (big sur 11.4
) . The cursor "leaves" supposedly on the second monitor, but there is a black screen. When you turn on the second monitor, it goes into power-saving mode (no signal) after a short time. If I change the settings of the second monitor, they are reset after a while. But everything works on the same imac under Windows 10 installed via BootCamp. Windows sees the monitor, allows you to work with it and change the settings as you want.