Paul,

Graphics card: NVIDIA Quadro K620
Graphics driver: NVIDIA 2015-04-26 / 9.18.13.4807

The original graphics driver was from December 2014 so we tried updating to the version above, without any change.

OS: MS Windows 7 Pro

Hub: Multi Stream Transport (MST) Hub DisplayPort 1-2, CSV-5200 MST 1-2 DP Hub
http://www.club-3d.com/index.php/products/reader.en/product/multi-stream-transport-mst-hub.1649.html

Display/screens/setup:
Graphics card->DP->Hub
1 x Dell UltraSharp U2412M (Hub->DP)
1 x Samsung SyncMaster 2443BW 24″ (Graphics card->DVI)
1 x Samsung SyncMaster 2443BW 24″ (Hub->DVI-adapter->DVI)

All set at 1920×1200 resolution.

DVI-Adapter: Dell-adapter provided with the computer – model number DANARBC084. The DP connector of this one has a “DP++” logo on it, if that’s interesting:
http://www.ebay.com/itm/331538473249?item=331538473249&lgeo=1&vectorid=229466&rmvSB=true

We also tried an older DP connector with same result:
http://accessories.ap.dell.com/sna/productdetail.aspx?c=au&l=en&s=dhs&cs=audhs1&sku=450-14017