I think I gots a patch:
/wp-admin/ms-sites.php
line 578
remove: $actions['backend'] = "<span class='backend'><a href='" . esc_url( get_admin_url($blog['blog_id']) ) . "' class='edit'>" . __( 'Backend' ) . '</a></span>';
Add: $actions['backend'] = "<span class='backend'><a href='" . esc_url( get_home_url( $blog['blog_id'] ) ) . "/wp-admin/' class='edit'>" . __( 'Backend' ) . '</a></span>';
This seems to fix it and points to get_admin_url being the problem.