Hi,
MacBook Pro (Retina, 15-inch, Mid 2014) has two thunderbolt/display ports on the left hand side just in front of the power connector. I have two Apple DisplayPort to DVI adapters to DVI cables directly to the two monitors, no other hardware in the way.
Only apps running in the taskbar/backgroun are Dropbox & iStat. Have tried with other apps open/closed, no change in UE behaviour. All other apps work on any of the monitors and have no issues: Photoshop, Android Studio, Xcode, Chrome.
Kind Regards
Kate