I’ve reported this via the game bug report system, but just so others are aware of it, as it took us a while to work out what was going on …
- We have guild events on the calendar.
- If someone signs “tentative” to an event, but then later uses the “remove” button to remove their tentative sign-up (usually because they want to accept instead, and there is no way to change their status from tentative to accepted without first removing their sign-up completely) the event disappears completely from their calendar.
- /reload or a relog brings the event back again.