We also have the new 7400. We are seeing a similar issue where the system screen just goes black. With the system set to "do nothing" when closing the lid, if you re-open the lid, the screen is black. The backlight is on but no output from screen. Sometimes happens when disconnecting from the dock as well (WD19).
I am running into a similar problem. Our brand new Latitude 7490s are connected to WD19s after we had a very high failure rate on the WD15s. The WD19s are losing ethernet connections pretty consistently and occasionally USB connections and rarely others, such as the display. These issues don't seem to happen when the users are working on the machine, rather after a long period of inactivity and the machine has gone to sleep or after a restart. The only fixes we have found so far are to unplug and plug the dock back in to the thunderbolt connection on the laptop or to perform another restart on the machine.
Edit: We are also running the latest firmware and BIOS on these machines.
We have several 7490 and brand new 7400 laptops and we are using the WD15 and WD19 docks with them. There is a rather annoying issue that we've been seeing where someone will try to dock the laptop and the light on the USB-C plug does not come on and the displays will not come on. Occasionally we will also see a message on the laptop stating that the incorrect power supply is connected. The only "fix" that we have found for this is to take another known working WD15 or WD19 and connect it to the laptop and then reconnect the non-working dock and it will light up and work normally. I have the latest BIOS and firmware updates on all the computers that I am seeing the issue with. Has anyone else come across this and found a permanent fix? Thanks in advance.