Unittest fails when executed from root folder

I have a lot of testcases in various subfolder. When executing all testcases from the root-testcase-folder one testcases fails. Nevertheless when executing this failing testcase seperatly it succeeds.

I get following errors during executing the testcase, which are non-related to the testcase itseelf.

Testing started at 22:07 ...
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Python32\lib\multiprocessing\forking.py", line 371, in main
    self = load(from_parent)
ImportError: No module named test_runnerServer


test_runnerServer is the file where the testcase is implemented in.
Any plans why executing the tests fail here?

Robert