[Solved] Ok.com Authorization doesn't work

I ran into a problem with Ok Authorization:

1. Created OAuth application on Ok.com with added  https://example.com/wp-admin/admin-ajax.php as a redirect URI.
2. Got Application ID, Public Key and Security Key.
3. Input these data to plugin settings and saved.

Authorization starts working good but when page redirects to /wp-admin/admin-ajax.php + long URL-parameter, user sees such an error:

This page isn’t working example.com is currently unable to handle this request.
HTTP ERROR 500

And when user manually goes to the page with comments there is such notification above the comment form:

Odnoklassniki authentication is unsuccessful (OAuth state does not exist).

How this problem can be fixed?