Okay, I have done some further debugging into what is going on:
My wife and I both have Samsung Galaxy S5 phones (SM-G900V) running Android 6.0.1. We’re both using the stock Calendar application with DavDroid and CalendarColors also installed. I also have OpenTasks and TeamCalendarSync installed, she does not. On my phone, I have the system calendar, an exchange calendar, and three CalDAV calendars.
When we first noticed the problem, it manifested by every time we would try to add a new event, the Calendar app would immediately crash. To further investigate, I force quit the Calendar app, cleared its cache and data, then started it again before trying to add an event. The event creation dialogue would then open without crashing Calendar. The calendar selected for the event in the dialogue was not one of the calendars being managed by DavDroid. If I attempt to switch to one of those calendars, the option radio button will not update to the desired calendar. However, if I cancel out of the calendar selection option, the event dialogue will show up as the desired calendar. The color for the event in the event creation dialogue will remain the color of the previously selected calendar. If the color chooser to clicked, the Calendar app crashed (this is the known and expected bug in Calendar). If the event is created without interfering with the event color, it will get created and pushed as normal, with its proper calendar color. However, the Calendar app will then remember the calendar managed by DavDroid as the last calendar for creating an event. When a second attempt at creating an event is attempted, the original behavior of immediately crashing Calendar is exhibited.
To add a new event through the phone, one must clear the Calendar app’s data and cache before each add. Also, this only works if the user has at least one other calendar on their phone that isn’t managed by DavDroid and Calendar will choose as the first event calendar at startup. Calendar’s selection at startup does seem consistent and deterministic.
I am a fairly technical user, so I can tolerate this workaround, especially since I can also add events via other clients on desktop and laptop. However, my wife is a non-technical user and only manages her calendars via her phone. From her perspective, “the calendar” is simply broken, and the DavDroid update is what broke it. If she didn’t have me, she would not have a workaround (which she finds onerous).
I have not submitted a bug against the Calendar app yet. I am not even certain where to properly file one, assuming one hasn’t already.