I have a Latitude E7450 running Ubuntu 15.04. When I connect an external display to it (either a beamer via the Displayport to VGA adapter or a Computer screen via DVI through the dock) the X system freezes. I can kill the session from a terminal (the terminal is displayed fine), log back in and again X windows freezes after a short moment of negotiating screen resolutions.
At some point it helped to quickly run
xrandr --output DP2 --mode 1024x768
before the freeze, then kill the session and logging back in. But when I tried this on a different beamer, the problem persisted. So I assume the issue has to do with determining the right resolution of the external device.
My workaround at the moment is to use my old laptop, but that is not what I bought the adapter and the dock for.