Redmine

I tried to use the plugin redmine_cas http://www.redmine.org/plugins/redmine_cas now.
After very simple config file edited, it's ok to login through CAS, but when it redirect to the service page, it add a ticket params in the url like

http://localhost/redmine/?ticket=ST-530-HRUbH1DP3wJzRP4SMS4A-cas

when i refresh the page, the ticket changed. The page display:

Internal error

An error occurred on the page you were trying to access.
If you continue to experience problems please contact your redMine administrator for assistance.

Back

But if i visit http://localhost/redmine, it can be visited. when I want to visit other page, it will add new ticket params in the url, and then display the same error.
I'm not good at ruby, please help me. Thanks so much!