I’m not sure what might have happened on your calendar on your server, but ICS Calendar can’t be at fault. The iCalendar (ICS) protocol is inherently read-only; ICS Calendar has no way to write anything to the calendar server.
I’m not sure how CalDAV may fit in here — ICS Calendar doesn’t support CalDAV at all. As for the Basic Authentication… we’re still dealing with an iCalendar subscription, so again it’s inherently read-only. There’s no way to write anything to the server.