There is at least one server using The Arena scene as a main and only battleground of the rotation. If you are willing to host a similar solution nonetheless, then see this. If you insist on utilizing a dedicated server and are sure that everything else is set up correctly, do the following.
- Open Sample_Deathmatch.txt file in your server directory,
- delete all lines beginning with "add_map" and "set_map",
- add a line "set_map multi_scene_17",
- save the file and send its modified version to the server,
- restart the server,
- connect to it and see the result.
Multi_scene_21 is not The Arena. It was a code name for Cold Coast, which eventually did not make it to Warband with 2016 Invasion Mode update.