Details of the exception is here http://orchard.codeplex.com/discussions/404336#post947301
To be more clear than in the upper thread, her are the steps
1) create a taxonomy named Section and add several terms in a 2 levels hierachy
2) Add a field based on this taxonomy to the Page Content Type
3) Create a PAge content and publish without selecting any value for the previous field
4) check the logs and see that without any other warning NHibernate is crashing because a parameter is not defined.