custom-server-side-field-validation.php

<?php

add_filter( 'ninja_forms_submit_data', function( $form_data ){
  
  if( ! my_nf_validation( $form_data ) ) { // Add check here.
    
    $errors = [
      'fields' => [
        '12' => __( 'An unexpected error occurred.', 'my-plugin' )
      ],
    ];
    
    $response = [
        'errors' => $errors,
    ];
    
    echo wp_json_encode( $response );
    wp_die(); // this is required to terminate immediately and return a proper response
  }

  // If no errors, be sure to return the $form_data.
  return $form_data;
});

custom-server-side-form-validation.php

<?php

add_filter( 'ninja_forms_submit_data', function( $form_data ){
  
  if( ! my_nf_validation( $form_data ) ) { // Add check here.
    
    $errors = [
      'form' => [
        'my-error' => __( 'An unexpected error occurred.', 'my-plugin' ),
      ]
    ];
    
    $response = [
        'errors' => $errors,
    ];
    
    echo wp_json_encode( $response );
    wp_die(); // this is required to terminate immediately and return a proper response
  }

  // If no errors, be sure to return the $form_data.
  return $form_data;
});

custom-server-side-validation.php

<?php

add_filter( 'ninja_forms_submit_data', function( $form_data ){
  
  if( ! my_nf_validation( $form_data ) ) { // Add check here.
    
    $errors = [
      __( 'An unexpected error occurred.', 'my-plugin' )
    ];
    
    $response = [
        'errors' => $errors,
    ];
    
    echo wp_json_encode( $response );
    wp_die(); // this is required to terminate immediately and return a proper response
  }

  // If no errors, be sure to return the $form_data.
  return $form_data;
});

error-format.php

<?php

// Form Error
$errors[ 'form' ][ $error_id ] = 'Form Error';

// Field Error
$errors[ 'fields' ][ $field_id ] = 'Field Error';

Am I missing something, the "Submit" button continues to show "Processing" after the JSON returns. (NF 3.3.7)

The same for me.

Same for me :(

Same for me...

The same was for me.

I found out, that you should return the response and without JSON encoding.
Also I was unable to display "form error", just "field error":

add_filter(  'ninja_forms_submit_data', 'my_ninja_forms_submit_data' ); 
function my_ninja_forms_submit_data( $form_data ) {
  $errors = [
    'fields' => [
      '5' => __( 'An unexpected error occurred.', 'my-plugin' ),
     ]
  ]; // '5' is field's id.
  $response = [
    'errors' => $errors,
  ];
  return $response;
}

Does anyone else get "Call to undefined function my_nf_validation()" error message?