This is coming from PyCrypto, a cryptography library used by Review Board (prior to 2.5.10) and Power Pack (prior to 1.4.4). It can occur if you have two versions of PyCrypto installed, one from your Linux distribution's package manager and one from the Python package manager (pip or easy_install). This has been encountered on Red Hat Linux and derivatives.
To solve this, you need to uninstall the version from your distribution's package manager. If you're on a Red Hat-based system, you can run:
$ sudo yum remove python-crypto
This should get you back up and running.