Cant see anything off the bat that would mess with it.
To test it, do the following:
Unload all plugins (o.unload *)
Make sure the arena is fully despawned. If it isn't, run the command I mentioned above.
Adjust the config of SurvivalArena so that it only requires 1 player to join ("Minimum players required for the event to proceed": 1)
Load EventHelper (o.load EventHelper)
Load SurvivalArena (o.load SurvivalArena)
Start the game with /survivalarena. Join the game and wait for it to begin.
Once the doors are open, check the loot and see if it has spawned.
Let me know what you find.