For users with the OpenID plugin installed, any other plugin that uses
session_start() outside of a function that is called only when needed causes problems with OpenID. (See this issue on Google Code)
At this point, I have had to disable any plugins that improperly use
session_start() so OpenID will work properly, and that includes Zemanta. I and thousands of other users would greatly appreciate it if Zemanta's plugin were rewritten to only call
session_start() when it's necessary; that should prevent interaction and interference with OpenID and other plugins with similar issues.