I have Ubuntu 11.04 I am trying to install Xvfb and CutyCapt in order to take a snapshot of webpage I followed a instruction https://github.com/jaequery/cutycapt-installer-scri
Had the same issue, solved it by running this instead:
xvfb-run --auto-servernum --server-num=1 [your script]
--auto-servernum : Try to get a free server number, starting at 99, or the argument to --server-num