As stated in the Changelog for v4.3.1 the handling of the menu entry at the admin page has been changed:
Core: Added specific position to ACF menu item to reduce conflicts with 3rd party plugins.
This seems to result in an incompatibility with plugin 'CMS Tree Page View'. Clicking on menu entry 'Custom Fields' now displays a 404 error page. After disabling 'CMS Tree Page View' this menu entry works again properly.