Hello Becky,
That is very, very strange.
Just to be sure, I know you already answered this, but you have definitely deactivated all plugins to rule out a conflict?
Here are some other things to try:
1. Have you tried reverting to the Twenty Eleven theme to see if it’s something with your theme?
2. Try setting your permalinks to ‘postname’ in Settings > Permalinks?
3. Try refreshing your permalinks by visiting Settings > Permalinks (it does it automatically)?
4. Try increasing the PHP memory for your website: http://www.dailyblogging.org/wordpress/increase-wordpress-memory-limit/?
Honestly if I were you and none of the above work, I would try to uninstall and then re-install the plugin. You may want to do this on a test environment to make sure you don’t lose any changes.
Does this happen any where else on your site, or only on the calendar settings?
And you are using the latest version of all our plugins and add-ons?