I'm having the same problem. My server runs both Nginx and Apache (if necessary, I use Nginx to proxy requests to Apache), with the site running under Nginx. Whenever I try to do anything with tags, I get the same error as previously mentioned.
I have root access to the server. I've looked at the logs, but nothing seems to shed any extra light on the problem. I'd imagine it's an issue with authorization, as discussed in a previously-linked-to thread Unable to make or create any posts.
