CloudCompare fails to launch with error: “QXcbConnection: Could not connect to display :0”

In the last week, CloudCompare has suddenly stopped launching. When trying to launch via desktop icon, nothing happens. When trying to launch via commandline (e.g. $ cloudcompare.CloudCompare) nothing happens and I get the following error: QXcbConnection: Could not connect to display :0 Aborted (core dumped) Trying to launch CloudCompare with sudo/as root user returns a slightly different error message: mkdir: cannot create directory '/run/user/0': Permission denied QXcbConnection: Could not connect to display :0 Having looked around Github, here and various other forums, this seems to be a problem that people have encountered with other programs that use Qt and/or OpenGL (see 1 , 2 , 3 ). I also realise that this isn't really enough information to debug the problem, so if there is any other diagnostic info I can/should provide, let me know and I will add it. My Environment -------------- - **OS & Version:** Ubuntu 14.04 LTS - **CC Version:** v2.8.1 (latest) - **Graphics card:** Intel Haswell Mobile - **Qt version:** 5.2.1