I have two U2715H monitors connected via one mDP->DP cable each to an E-Port Plus docking station (actually a EURO2 Advanced Port Replicator) in which is seated a Latitude E7450 laptop. All items are brand new. The E7450 is equipped with 16 GB RAM and two graphics cards: Intel HD Graphics 5500 and NVIDIA GeForce 840M; selection is automatic, which usually means the 5500 is in operation. It runs Windows 7 Enterprise SP1 (64 bit). The docking station apparently has DP 1.2a capabilities (for daisy chaining) but DP 1.2 is disabled in both monitors.
I have successfully set up the system to have the Windows desktop span both monitors at 2560x1440 resolution, even with the Latitude's internal screen as a third extension at its full resolution (1920x1080). The Main display may be set to either of the two (or three) and I can successfully move the sequence of the monitors around (left, middle, right).
So there is no doubt about the system's capability to support the intended configuration. However, when I dock the Latitude (mostly with the lid shut) and boot it, most of the time it cannot reestablish the configuration of the two external monitors. Some of the symptoms are: One or both monitors go black, indicating cable connected but no signal arriving from the docking station. If there is a picture on one of the monitors at startup, it may or may not contain the login screen. (I can still log in because I know what keystrokes to send.) If the login screen is visible, it is usually in a low resolution. If I have several screens active, but one is black, I can move the mouse pointer (or an entire window) across the black screen (in the middle or on the side) but it is not visible there.
Both monitors come straight from the box and the only change I have done is to switch from DP to mDP as input channel. I have reseated the cables and swapped the cables between the two DPs on the docking station. I have turned the monitors off and on. I have run the self test as prescribed on both monitors. The laptop connection on the docking station is a bit wobbly but I am using the supplied spacer for the E7450 and making sure the connection is correct and stable. I have updated all drivers and the E7450 BIOS. I have recently seen a reference to a BIOS for the docking station; I didn't know it existed, but I cannot find it.
The instructions packed with the monitors had a note about consulting the supplied CD for using the monitor at maximum resolution. The E7450 doesn't have an optical drive but I have read all the documentation for the monitor at Product Support without finding anything to illuminate the situation (beyond making sure that the graphics card and the drivers are up to the task).
Once I have logged in, I can manage to get everything back in order by randomly(?) reducing screen resolution, changing the position of the main display (left-middle-right), deselecting and reselecting mDP as input channel, and forcing other changes. Once both or all three screens are in operation, I can set them all to maximum resolution and start working. However, this is rather bothersome and I would like to have a stable setup.
The configuration seems to survive power cycling of the laptop/docking station but not if I also undock the laptop. I should mention that when I undock the E7450, I may be using it in other configurations with or without other external monitors through other ports, before I turn it off and later re-dock it as described. However, for the first month I had two older non-Dell monitors of lesser resolution (on DVI and VGA ports) connected to the docking station and the configuration was stable, and for years I have been used to my older Latitude with an older docking station remembering its peripherals between different hardware setups.
Please, anyone - and Dell support staff in particular?
(Edited/updated after move from Forum/Peripherals/Monitors to Forum/Laptop/Video)