After Mac update to OS X Yosemite Version 10.10, I can’t run Xephyr.
Xephyr cannot open host display. Is DISPLAY set
I have tried to fix it by “export DISPLAY=:1”, or something else. However, no approach to fix it. I tried to update X11 before giving up.
Ah, it worked!
I remember downgrading X11 also solved another problem several monthes ago. So update or degrade your X11 might be a way to solve your problem.