Installation problem: ImportError: No module named _weakrefset

pyCharm installed w/o errors (on Linux Mint) and the initial session (config etc.) was error-free. However subsequent calls to pycharm.sh result in the following error:

./apps/pycharm-community-4.0.4/bin/pycharm.sh
Traceback (most recent call last):
  File "/usr/lib/python2.7/site.py", line 68, in <module>
    import os
  File "/usr/lib/python2.7/os.py", line 398, in <module>
    import UserDict
  File "/usr/lib/python2.7/UserDict.py", line 83, in <module>
    import _abcoll
  File "/usr/lib/python2.7/_abcoll.py", line 11, in <module>
    from abc import ABCMeta, abstractmethod
  File "/usr/lib/python2.7/abc.py", line 8, in <module>
    from _weakrefset import WeakSet
ImportError: No module named _weakrefset

My system is up-to-date and python-virtualenv is installed.
Can anyone give me a tip how to resolve this error?