Nginx + WordPress : 414 Request-URI Too Long
Solution 1:
From somewhere on the internet:
Most likely you are trying to configure client_header_buffer_size/large_client_header_buffers in a pure virtual server{}. This won't work as request headers parsing happens before Host header is known (and virtual server isselected), hence parseing happens in a context of the default server for a listen socket.
You have to configure client_header_buffer_size/large_client_header_buffers in a default server (or at http level).
Solution 2:
Found the guilty one... Better WP Security Plugin !
There is an option to limit the URLs length.
However, this plugin is amazing but adds many tweaks to protect a Wordpress installation by default.
Thanks all for your help!