Server rollbacked 6 hours

So as the title says, the server suddenly closed by itself and didn't save anything from the last 6 hours of progress. Is this a common occurence? We don't feel like losing all the progress of 3 players in such long time again.

Only 3 people playing, we rarely get 4 players in at the same time. The backups are all good but what happened is we were just doing mundane tasks (mining and walking around the caves) and the server forced a disconnection without doing the usual save when the server closes. We even had the usual [The game has been saved.] every 30 minutes.