When connect HP Elitebook 840 G5 with LG 27UL850-W, screens start flashes

[Rafail]

Hi! I have LG 27UL850-W monitor. When I connect it with type-c (thunderbolt) cable both laptop and LG monitor starts to connect and disconnect, like flashes. With HDMI connection, it works fine. What is the problem?

I found that when I have already connecting LG monitor with the laptop via HDMI and then connect type-c and disconnect HDMI, it works fine then. But if it goes to sleep mode and wake-up, the problem appears again.

[soccer_dan]

you may be seeing a problem with the usb-c PD (Power Delivery) power coming from the display via the usb-c cable... it may not be 100% compatible, or you may need to adjust the sleep settings in the monitor... Did you try plugging a power supply to the laptop while also connecting to the LG via usb-c? does is still flicker?

also, is your system BIOS and drivers up to date? use HP Image Assistant to confirm

[soccer_dan]

actually, the issue is most likely that the G5 generation usb-c/tb port supported (in usb-c mode) alt-mode video out at DisplayPort 1.2 speeds, which does not support UHD/4k resolutions... That resolution requires a DP1.4 version, now on newer devices