Ok, I think I found the issue. Some of this was my fault.
The latest issues (blank page when posting or logging out and attachment wankery) were caused by a debug output I had put in that wasn't allowing redirects to happen after it (basically, it caused problems once the other stuff was fixed). I removed the output and I think it's redirecting normally now.
Is anyone else still (as in since this post) seeing blank pages or having other trouble?