This usually occurs if you have uploaded your own world or if you have up/downgraded the number of slots you have. To resolve follow the instructions below
The reason this happens is that Space Engineers uses the world settings over the main settings file so if the world was generated with less or more slots it will stay at that old value.
1. log into your game panel
2. go to game services > select your server
3. go to File Manager then navigate to Saves > your currently selected persistent world (folder)
4. find sandbox.sbc and click the pencil (edit) button
5. find <MaxPlayers>
6. edit the number to what it should be then save the file
7. restart server for new settings to take effect
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