I'm surprised no one has mentioned this yet.
It's possible WP Maintenance Mode is causing a problem because it calls pluggable.php too soon. See http://wordpress.org/support/topic/using-current_user_can-breaks-plugin and http://wordpress.org/support/topic/current_user_can-in-the-_construct
I'm checking to see if any other plugin could be at fault, will let you know if I find I'm wrong.
Thanks for an otherwise great plugin!