Well guys, the problem came back... I am really trying to have time to manage this forum, but it is difficult. So we lost another full month. The server went down and the redis (in-memory database) didn't have the time to commit to disk. The configuration is to write to file every minute if there's at least 10 keys changed, but it looks like it doesn't work for a mysterious reason.
I am no expert in redis and I am trying to find the time to update the forum... Not easy. If anyone have the time and the knowledge to import a redis database / nodebb forum I would be glad to export the installation to another server / admin.
(at least for now I am forcing a commit to disk of the in-memory database everyday (cron.daily) and doing a back-up in the cloud every night)...