Comment 6 for bug 72192

After the button is enabled, executing "xhost +local:" or the more safe "xhost +si:localuser:boinc" makes the graphics display.

See also http://wiki.debian.org/BOINC/Troubleshooting

I'll see if we can supply some patch, which will inform the user about the "xhost" issue when displaying the graphic failed.
According to the Debian wiki page mentioned above it's not a good idea security wise to execute this command by default (although I think that the second variant should be quite safe, as it only allows it for the local "boinc" user.