Public urls for public exhibitions are not working

I upgraded from Omeka 1.3.2 to 1.5.3 today. I also upgraded ExhibitBuilder to it's newest version. I have a public exhibit, but the url to the exhibit renders the following error: Not Found

The requested URL /digital/exhibits/show/nkk was not found on this server.

Has anyone encountered a similar problem or have any ideas for how I can resolve this?

Is the ExhibitBuilder plugin activated?

Upgrading Omeka and/or a plugin usually will automatically deactivate the plugin until you click an "Upgrade" button on the admin panel.