Cannot view existing calendars, HTML Error, and 500 Error

My users are getting a very strange mix of errors when trying to use team calendar, that make it unusable.

1) When my several developers go to Calendars, they are greeted with the error $calendarHtml

2) The others will see the calendar, but will not be able to add any existing calendars. The existing calendar lists will not be populated, and searching provides no results.

3) When deleting/removing a calendar, either by deleting the calendar from Confluence or by removing it from their view, users will get a 500 error (see screenshot).

I have noticed that calendar functionality with #2 may be linked to #3, as we were able to add calendars until one user removed several calendars, and then no more users who tried were able to add existing calendars.