Anonymous attempt to access path alias started with 'admin' is redirected to frontpage

How to reproduce:

1. Install and enable Pathauto;
2. Create a node with title "Administrative requirements", set "Generate automatic URL alias";
3. Try to access your node by aliased path anonymously (/administrative-requirements by default).

I found that Drupal is bootstrapped with empty ?q=. Everything works fine when node is accessed by /node/[nid] path, or after manually changing the first part of node's alias.

Please help me to find out which part of Nginx configuration could be responsible.