You can upload Space Engineer saves directly through Clanforge, here's how:
1. Log in to Clanforge
2. Click on your Server Name, or the small, blue "Manage" button to the right of the page
3. Open the World tab below the server info
4. Navigate to C:\Users\USERNAME\AppData\Roaming\SpaceEngineers\Saves\STEAM64ID\
4. Select your World folder on your local computer and rename it to 'Universe'
5. Add your Universe to an archive. It must be in either .zip or tar.gz format!
6. Select the archive you want to to upload from your local machine direct to your server
7. Enter anything into the Description box
8. Click Upload
9. Once uploaded open the backups dropdown inside of the World tab and select the most recent one (the one you uploaded).
10. Click on "Restore a Backup" to restore it. This will restart the server to apply it.
If your server starts however reports 0/0 players then you have a configuration issue. This is usually caused by a miss-match between the world's scenario that is set in the Universe.sbc and the one you are using in the SpaceEngineers-Dedicated.cfg file on Clanforge. If you make sure they both match then it should start up fine!