I've ran into this with different HP PC's and docking stations using DisplayPort, but only with non-HP branded monitors, such as some 24' 1080P IPS Viewsonics. Happy users are important.as we all know. It may not be the perfect solution, but it works, and now that user is able to run 3 extended monitors, instead of just two that do not work through the DisplayPort ports. The adapter connects to one of the DisplayPort ports, and then I can run up to 3 HDMI connected monitors through the adapter. My solution ended up being a DisplyPort to HDMI adapter (about $50). No idea what is happening on the back side of this, but evidence points to dual DisplayPort issues.probably with the drivers. So, it appeared to me to be something wonky going on when both DisplayPort ports are used at the same time.

If I only use one of the DisplayPort ports at a time, the monitor works fine.but as soon as I connect a monitor to both DisplayPort ports at the same time the video goes out again. I tested the monitors on another computer, and they worked without issue. It worked fine out of the box, but after about a month the video stopped getting to the monitors for some reason. I have an HP ProDesk 600 G3, with two integrated DisplayPort ports, that did the exact same thing.