I think i finally found the problem. A month ago I was playing around with a few vim plugins for python development. One of them seemed to place the file _thread.py at /usr/local/lib/python2.7/dist-packages/_thread.py. Through this it was available to python2.7 even though it was not meant to be used with it.
After I deleted the files /usr/local/lib/python2.7/dist-packages/_thread.py and /usr/local/lib/python2.7/dist-packages/_thread.pyc roscore is able to start without any problems.