When behind proxy, /synchrony location should have explicit intercept errors off. Sites with reverse proxy setups sometimes have "intercept errors on" to redirect to standard site-wide 404 page. This breaks "Create" page functionality in Confluence 6 with collaborative editing on, which apparently uses 404 call, a solution in this case is, i.e. for nginx:

    location /synchrony {
        proxy_intercept_errors off;

Issue discovered and tested on Confluence 6.0.5 behind Nginx.