Thank you for posting, and I would be happy to help with your UD-3900 docking station!
Thank you as well for the detail you provided that is much appreciated. The UD-3900 cannot affect the image on the connected display as you describe, and in similar the cases the root cause was determined to be the settings within the display itself.
To expand further, when a display is connected using a VGA video cable (which carries an analog video signal) as you describe it is sometimes necessary to manually adjust the image settings within the display in order for the image to appear correctly. HP speaks to these adjustment options on electronic pages 22-24 in your display manual -> http://h10032.www1.hp.com/ctg/Manual/c05105530
Based on your description, it sounds like you may just need to adjust the ‘vertical position’ setting that HP states is available. From pages 23-24 of the manual:
Press the Menu button on the bottom of the front bezel to open the OSD menu, and then select Image Position.
Press the Function buttons on the bottom of the front bezel that display up and down arrow icons to properly adjust the position of the image in the display area of the monitor. The Horizontal Position shifts the image left or right; the Vertical Position shifts the image up and down."
Please let us know if that information helps!
P.S. It appears that your HP 24es monitor has two video inputs, one VGA and one HDMI. An alternative to using the included DVI to VGA converter and the VGA video cable you are presently using would be an inexpensive HDMI-DVI video cable such as this -> http://amazon.com/dp/B014I8UQJY
The DVI end of this cable will connect to the DVI video output of the dock (the DVI to VGA converter will no longer be needed), and the HDMI end of the cable will connect to the HDMI video input of the HP 24es display. This cable will carry a digital video signal, so most likely the adjustments described above would not be necessary.