There was something that caused a short regular restart of the nginx reverse proxy in from of the forum. This is fixed now. The db crash is likely related to the regular backups. I temporary disabled the backups and in a few days we will know if it still crashes or not.
DB is back. Got corrupted again. We ordered new hardware and are currently preparing everything. So in the next weeks we will move everything to the new hardware and hopefully this DB issue is gone then.