After Mac update to OS X Yosemite Version 10.10, I can’t run Xephyr.

Error occurs

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.