[Plugin: Mailing List] Message box missing – doesn't work

When I installed the plugin, this message appeared: The plugin generated 193 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

The plugin still seems to work though, except for one critical error. It’s not possible to compose a new message to send. The “message” box is missing!