Some time back, (maybe a year?), my environment was working correctly. But some time in the last year or so I started getting "can't open display" errors.
It's weird because I can ssh into the ubuntu box, (from a mac, over cisco ipsec), and start several new x11 clients. But at some point, (1 - 20 minutes later), new attempts to start x11 clients result in "can't open display" errors. If there's any other antecedent event that leads to the "can't open display" problem, I havent' figured it out.
My DISPLAY hasn't changed in this time. Preexisting windows continue to exist and function so I believe my local X11 server is good, running, and that the previously existing X11 forwarding tunnels are still functioning. It appears to be a problem with the initial rendezvous to the display.
Does this sound familiar to anyone?