roscore won't run: "could not contact master"

edit

In this month I encountered the similar problem twice, one time with some other guy's ROS Fuerte under Ubuntu Precise 64bit, and now with my own computer under Ubuntu Precise 32bit... Just now I tried to run roscore but it could not start after some time of waiting, throwing out almost the same error messages:

... logging to /home/kai/.ros/log/8bd2c78c-3638-11e2-b312-14dae93037a4/roslaunch-Kai-System-27427.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://Kai-System:50291/
ros_comm version 1.8.10

SUMMARY
========

PARAMETERS
 * /rosdistro
 * /rosversion

NODES

auto-starting new master
Exception AttributeError: AttributeError("'_DummyThread' object has no attribute '_Thread__block'",) in <module 'threading'="" from="" '="" usr="" lib="" python2.7="" threading.pyc'=""> ignored
process[master]: started with pid [27448]
ERROR: could not contact master [http://Kai-System:11311/]
[master] killing on exit
Traceback (most recent call last):
  File "/opt/ros/fuerte/bin/rosmaster", line 35, in <module>
    rosmaster.rosmaster_main()
  File "/opt/ros/fuerte/lib/python2.7/dist-packages/rosmaster/main.py", line 73, in rosmaster_main
    configure_logging()   
  File "/opt/ros/fuerte/lib/python2.7/dist-packages/rosmaster/main.py", line 57, in configure_logging
    _log_filename = rosgraph.roslogging.configure_logging('rosmaster', logging.DEBUG, filename=filename)
  File "/opt/ros/fuerte/lib/python2.7/dist-packages/rosgraph/roslogging.py", line 86, in configure_logging
    rosgraph_d = rospkg.RosPack().get_path('rosgraph')
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 162, in get_path
    self._update_location_cache()
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 145, in _update_location_cache
    list_by_path(self._manifest_name, path, cache)
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 57, in list_by_path
    for d, dirs, files in os.walk(path, topdown=True, followlinks=True):
  File "/usr/lib/python2.7/os.py", line 294, in walk
    for x in walk(new_path, topdown, onerror, followlinks):
  File "/usr/lib/python2.7/os.py", line 294, in walk
    for x in walk(new_path, topdown, onerror, followlinks):
  File "/usr/lib/python2.7/os.py", line 294, in walk
    for x in walk(new_path, topdown, onerror, followlinks):
  File "/usr/lib/python2.7/os.py", line 284, in walk
    if isdir(join(top, name)):
  File "/usr/lib/python2.7/genericpath.py", line 41, in isdir
    st = os.stat(s)
KeyboardInterrupt

My ROS configurations has not been touched since last time I run it(no more than a week), so I also think the problem lies in somewhere else.

I will try rebooting and stopping vpns to see if it helps.

Update: Seems this is not a easy to solve problem. The last time I asked that guy to re-install a 32bit ubuntu and the problem disappeared, but this time I don't think this is a good idea. More info: I did not use "rosws" but the old-fashioned way of configuring ROS env. Here is a list:

kai@Kai-System> export | grep ROS
ROSLISP_PACKAGE_DIRECTORY=/opt/ros/fuerte/share/common-lisp/ros
ROS_DISTRO=fuerte
ROS_ETC_DIR=/opt/ros/fuerte/etc/ros
ROS_MASTER_URI=http://localhost:11311
ROS_PACKAGE_PATH=/home/kai/Projects:/home/kai/ros_stacks:/opt/ros/fuerte ...