doh, I only searched for open bugs. Not for closed.
This ticket is a dublicate of http://bugs.python.org/issue1471427
and fixed in python 2.5.
If somebody has similar problems, here is a quickfix:
I finally was able to reproduce the issue. It only happens when the path
without the filename but the trailing slash is exactly 100 chars long.
Then, because of the trailing slash, tarfile makes this thing a
directory, and if the file itself was not empty, the next read cannot be
parsed as a tar file. Since I am bound to 2.4 I will rename the directories.