aborted games by server without reason in last round of swiss tournament

tournament:
lichess.org/swiss/5dUCOdNW

four games were aborted before or in the first move.
All players were online.
All games were set as draw.

What happened?

Server restart?

server restart was before.
And only four of eleven games had this problem.

Maybe 7 games refreshed and 4 did not do so timely?