HD 4400 graphics Displayport 1.2 support

Hello,

I recently started to experience problems with DP 1.2 support on my Latitude E5440 with Intel HD 4400 graphics. At firs the monitors failed to show image when I docked my laptop. Undocking and docking again would then resolve the issue. But after a few days it failed to show image no matter what. After long hours of troubleshooting with drivers and firmwares and updates etc I discowered  that disabling the DP 1.2 from my monitor enabled me to get image at least to one of my screens (diabling can only be done when there is an actual image on the screen?! :P )

As I have now come to an understanding through multiple support requests that it is actually something that is not supported under WIN 10 with my setup.

My WIN 10 is an update from WIN 7. Could this be the cause that it was working in first place? I cant confirm it because it was some time ago but I started to use the DP 1.2 option when I had already updated to WIN 10.

Does anyone have an idea of such similar cases or have I been privileged to use something that was not supposed to work in the first place?

My setup:

Dell Latitude E5440

WIN10 upgraded from WIN7

2x Dell U2414H monitors daisy chained

Dell dock

All the best,

Vahur