Rarely, my system can't run any new graphical application and gives me errors like "Cannot open display"

Without apparent reason, my system sometimes goes into a very strange system, in which I can’t open any new graphical program (including, firefox, gimp, okular) from the programs that are already opened. If I open a new terminal, then in this new terminal everything is back to normal in this new terminal, but all others terminals/programs that were opened before still have this strange behaviour.

More informations can be found in the bug I filled here.