Valid ballot created before election closed is discared if received after.
If all voters vote, then the last voter might risk having their ballot discarded, because ballots are sent sequentially. Servers close when receiving all ballots. This forces all servers to close, when enough have closed. However servers can be forced to close before having received their ballot from the last voter.
This result in the ballot only being tallied on the servers, who agreed to close. This can be fixed by giving each ballot a timestamp, and checking this timestamp is before closing time. Then have a delay between closing and tallying to let late ballots arrive. After tallying have begun ballots should be discarded.