After playing for about 10 minutes, the server stopped unexpectedly (crashed) after the two of us started exploring old and new parts of the world. After the crash, restarting the server did not work as it would crash after starting again (and having a person attempt to log in).
View crash reports attached. The world already exists since Minecraft Beta 1.6.
Issue seems to be resolved after force upgrading the entire world using --forceUpgrade