Before using DF I had an issue in windows10 where I would unlock my running laptop and all windows from the second monitor would go to my laptop - very very annoying.
This did not happen all the time but once or twice a day.
Thought that I would use DF again so I fetched the upgrade to 8 and I run it portable (laptop from work - can't install anything).
Now however, it happens nearly everytime I unlock the laptop again, all screens switched from the secondary monitor to my laptop.

I googled the issue before and it might have something to do with windows changing resolution after unlocking.
http://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/windows-7-movesresizes-windows-on-monitor-power/1653aafb-848b-464a-8c69-1a68fbd106aa?auth=1
Does DF change the resolution as well when unlocking? There is a clear difference in using the standard functionality in win10 and DF.

It feels like my secondary display is plugged out each time, but I checked the cables numerous times.

Any ideas?

Sorry, not using DP but VGA, still very good advice, didn't know they did that.
One key thing I probably should have mentioned is that I'm using a lenovo docking station.
Regardless, I tried the settings that are suggested in the link but my windows still switched screen and stayed there.

So I concluded that the windows must somehow switch screen after or right after I lock.
I therefore changed the setting to save window positions when I lock.
This workaround works, thanks!