To understand why your redirect isn’t working, you should consider the mechanism it’s using: the Location HTTP header. The problem here is that HTTP headers cannot be sent once you begin outputting content onto the page. Your task is to determine where that content is being output.
- Is it a plugin? Try disabling all your plugins and see if that fixes it.
- Is it your theme? Try switching to a different theme (modify it to use your logout hook) and see if that fixes it.