Well, found the problem myself.
The problem is here: Portal settings -> General -> Default Landing page
I configured this setting to page which is not accessible to Guests. And permissions of default landing page was applied to url /robots.txt.
It's really unclear feature.
Seems for me it bug. Because help tip says:
This is the page that users are automatically redirected to after logging in. For example, if you want the default landing page to be http://localhost:8080/web/guest/login, then set this to /web/guest/login.
But guests are not logged in.
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