Hi Nigel, very useful !!
I turned DP 1.2 off and it s now working with both DP to mDP and also with a mDP to mDP cable I did buy meanwhile. So now I can connect the Mac Pro to the U3415w with Hdmi (50hz) and to the DP or the mDP ports. (60hz).
The next problem I have now, is that I can only use 3440x1440 and 2560x 1280 or somethng similar. Wen I try to use PBP, both left and right screen has those standard resolutions , and of course I can't use them on a smaller screen. Did you find any way to configure say a 1720x1440 or something similar ? Thanks , giovanni