When creating a new entry for a CALDAV calendar, sometimes it happens that after Save&Close nothing is stored. Everything you entered into the creation window is lost. After doing the same work again, the new entry will be stored and shown in the calendar view.
The reason might be a slow answer of the CALDAV server. It uses a hard disk that is stopped when idle. It may take a few seconds to start it again. Thunderbird should wait for the server, or at least it should return to the creation window showing the data that has been entered before.