Hello, @nuriaher
May I know if you have tried to stop and start the container using these commands to verify that the instance is up again?
docker stop mattermost-preview
docker start mattermost-preview
It worked for me, so it should work on your end too.
Take note that the preview mode is using an older version of Mattermost:
I would recommend you to check on the Production Docker Deployment if you are planning to explore more on the latest version of Mattermost 5.17.1: