I'm having trouble with one blog displaying the Dashboard /wp-admin/index.php page. No other blogs are experiencing this problem that I'm aware.
Other pages work such as /wp-admin/edit.php or /wp-admin/themes.php. The main index.php will timeout with a 504 error.
This error shows up in the log:
(104)Connection reset by peer: core_output_filter: writing data to the network