Try adding the following just below the server_name directive:
# Add trailing slash to */wp-admin requests.
rewrite /wp-admin$ $scheme://$host$uri/ permanent;
Edit: I misread your issue, though you might still want that line in there. I've had issues with nginx and WP under a subdirectory.
# Add trailing slash to */wp-admin requests.
rewrite /wp-admin$ $scheme://$host$uri/ permanent;
Edit: I misread your issue, though you might still want that line in there. I've had issues with nginx and WP under a subdirectory.