This could be due to a number of factors. Please try/check the following:
1. Are you running the latest version of WordPress.
2. Are you running the latest versions of our plugins (2.0.11)
3. Have you tried deactivating all other plugins to rule out a conflict?
4. Have you tried reverting to the Twenty Eleven/Twelve theme to see if it's something with your theme?
5. Try setting your permalinks to 'postname' in Settings > Permalinks
6. Try refreshing your permalinks by visiting Settings > Permalinks (it does it automatically)
7. Try changing the event template in Events > Settings > Template.
8. Try changing the time zone to a UTC zone instead of a city.
Let me know whether or not that helps.