Latitude E7440 with E-port doesn't detect monitor via DP

• Dell Latitude E7440 with E-port docking station doesn't detect monitor ( 2560x1440) via displayport.  The only way to get the docking/computer to detect the attached display is to remove the powercable from display and power-on again.

Windows 7 installation, all drivers updated, bios A15, HDMI works fine but is limited to 1920x1200.

I checked that the A05 Synaptics firmware is present.

We have several computers With the same problem at work.