Link coming from Mail Notification breaks Calendar.

Problem

When you receive a notification from Team Calendars:

You are receiving this notification because you are watching the following:
Vacations

It will present your link to view them, and will be asked to add this calendar to your with an "Add it now" link.

When you click this link, it will automatically break all your calendars, and you'll be presented with:

(-) Error(s) occurred.
    500 - 

There is nothing in the logs

Steps to reproduce (Assuming you already have a working mail server)

1. Install Confluence,
2. Install Team Calendars
3. Create a calendar
4. Create an event, and add this event to a user
5. This user will receive an mail notification, open it, and the link should be something like:
   • http://heck.com:8090/calendar/previewcalendar.action?subCalendarId=b8f3a4e6-2065-4400-8cd4-f9347e651730
6. Add it now
7. Voila! 500 error page forever.

Workaround

The workaround as much as it is not perfect it allows the user to still browse the calendar, just access the calendar through the link provided on the mail notification