If you set up a Google Mail account on Windows Phone 8 and later decide
you want to also sync your contacts and calendar, here's how to do
Calendar icons. Those little symbols on your calendar provide quick info about details like whether you've accepted an invitation, scheduling conflicts, and ...
Learn how to create appointments, sync calendars,
and more on your Windows Phone.
Workaround: I was pointed to a workaround in the comments below: Don't use CalDAV :) Switching to EAS (set up instructions here) seems to have fixed the new calendar event creation issue. At least