On my very basic Drupal installation (default theme, Croatian/Bosnian/Serbian languages, AddToAny and Social Media Links block extensions) after upgrading from 8.2.6 to 8.3.0-rc1 or rc2 I get:
Got error 'PHP message: Uncaught PHP Exception InvalidArgumentException: "Field is unknown." at /var/www/zakon.link/html/core/lib/Drupal/Core/Entity/ContentEntityBase.php line 509\n
upon accessing a static page. Downgrading to 8.2.7 fixes the problem. This is all that is printed in PHP-FPM log. Any other info I can provide?