WP User Manager - Remove nickname validation from the account form

wpum_remove_account_nickname_validation.php

<?php


/**
 * Remove an object filter.
 *
 * @param  string $tag                Hook name.
 * @param  string $class              Class name. Use 'Closure' for anonymous functions.
 * @param  string|void $method        Method name. Leave empty for anonymous functions.
 * @param  string|int|void $priority  Priority
 * @return void
 */
function wpum_remove_object_filter( $tag, $class, $method = NULL, $priority = NULL ) {
	global $wp_version;
	$new_wp_filter_struct = false;

	$filters = $GLOBALS['wp_filter'][ $tag ];
	if ( empty ( $filters ) ) {
		return;
	}

	if (version_compare( $wp_version, '4.7', '>=' ) && isset($filters->callbacks)) { // new 'wp_filter' structure (WP >= 4.7)
		$filters = $filters->callbacks;
		$new_wp_filter_struct = true;
	}

	foreach ( $filters as $p => $filter ) {
		if ( ! is_null($priority) && ( (int) $priority !== (int) $p ) ) continue;
		$remove = FALSE;
		foreach ( $filter as $identifier => $function ) {
			$function = $function['function'];
			if (
				is_array( $function )
				&& (
					is_a( $function[0], $class )
					|| ( is_array( $function ) && $function[0] === $class )
				)
			) {
				$remove = ( $method && ( $method === $function[1] ) );
			} elseif ( $function instanceof Closure && $class === 'Closure' ) {
				$remove = TRUE;
			}
			if ( $remove ) {
				if ($new_wp_filter_struct) {
					unset( $GLOBALS['wp_filter'][$tag]->callbacks[$p][$identifier] );
					if (count($GLOBALS['wp_filter'][$tag]->callbacks[$p]) == 0) {
						unset($GLOBALS['wp_filter'][$tag]->callbacks[$p]);
					}
				}
				else {
					unset( $GLOBALS['wp_filter'][$tag][$p][$identifier] );
				}
			}
		}
	}
}

add_action( 'init', function() {
	if ( ! is_user_logged_in() ) {
		return;
	}

	wpum_remove_object_filter( 'submit_wpum_form_validate_fields', 'WPUM_Form_Profile', 'validate_nickname', 10 );
}, 20 );

Save this file to your /wp-content/mu-plugins/ directory (you might need to create the mu-plugins directory).