There has been a new ZEO (Zope Enterprise Object) release, primarily made to achieve compatibility with code in Zope 2.3.1. If you are running ZEO, you need upgrade to ZEO release 0.5.0 before you can use Zope 2.3.1 reliably. The new release also fixes bugs found in previous releases, and can be used with Zopes prior to 2.3.1. For more information, see http://www.zope.org/Products/ZEO.