There are two common causes of this problem

Power Pack and Review Board are installed with different Python versions

You may be installing Power Pack for Python 2.7 while Apache is using Review Board on Python 2.6, or vice-versa. To fix this, you'll need to see what Python version you're using for Review Board and use the appropriate version of pip (pip-2.6 or pip-2.7).

A dependency of Power Pack is failing to load

A common culprit is PyCrypto. Your Linux distribution may be supplying a package for an older version that's conflicting with a newer version required by Review Board. See this article for information on how to resolve this problem.