Running a script at startup that uses xrandr to set display options
What you usually need to do in these situations is adding a small break in the command after login. Add this to your Startup Applications:
/bin/bash -c "sleep 15&&xrandr --output DFP10 --rotate left && xrandr --output DFP11 --right-of DFP10"
(If that is the normally working command)
The reason is that most likely the command is performed too early, while the desktop is still loading. It will break then.
You can experiment a little with the sleep 15 duration to see what is needed or possible.