It's a homelab, and a server nobody plays on for 7 days is deleted to free the slot. Don't let the server be the only copy of your world. Download one before any big change, and every so often anyway.
- Stop the server from the Console tab in the panel. Archiving files while the game is writing to them is how you end up with a broken copy.
- Open the Files tab and tick the box next to your save folder.
- Click Archive. The panel packs the folder into one file in the server root.
- Click the archive and choose Download. For a big world, download it over SFTP instead. Browser downloads of multi-gigabyte files tend to fail near the end.
- Delete the archive from the server once you have it, so it isn't taking up space.
- Keep the file somewhere safe. If the server is ever removed, your download is what survives.