Problem: Error executing query

Witam!
Co około pół godziny mam problem z witryną. Działa normalnie aż tu nagle wyskakuje taki komunikat:
Warning: mysqli_set_charset(): Error executing query in /home/marketinguzd/ftp/wordpress/wp-includes/wp-db.php on line 756

Warning: Cannot modify header information – headers already sent by (output started at /home/marketinguzd/ftp/wordpress/wp-includes/wp-db.php:756) in /home/marketinguzd/ftp/wordpress/wp-includes/pluggable.php on line 1219

za 5 minut działa normalnie.
Zamieszczam fragment kodu z wp-db.php linijka 756 jest wytłuszczona

/**
	 * Sets the connection's character set.
	 *
	 * @since 3.1.0
	 *
	 * @param resource $dbh     The resource given by mysql_connect
	 * @param string   $charset Optional. The character set. Default null.
	 * @param string   $collate Optional. The collation. Default null.
	 */
	public function set_charset( $dbh, $charset = null, $collate = null ) {
		if ( ! isset( $charset ) )
			$charset = $this->charset;
		if ( ! isset( $collate ) )
			$collate = $this->collate;
		if ( $this->has_cap( 'collation' ) && ! empty( $charset ) ) {
			$set_charset_succeeded = true;

			if ( $this->use_mysqli ) {
				if ( function_exists( 'mysqli_set_charset' ) && $this->has_cap( 'set_charset' ) ) {
					<strong>$set_charset_succeeded = mysqli_set_charset( $dbh, $charset );</strong>
				}

				if ( $set_charset_succeeded ) {
					$query = $this->prepare( 'SET NAMES %s', $charset );
					if ( ! empty( $collate ) )
						$query .= $this->prepare( ' COLLATE %s', $collate );
					mysqli_query( $dbh, $query );
				}
			} else {
				if ( function_exists( 'mysql_set_charset' ) && $this->has_cap( 'set_charset' ) ) {
					$set_charset_succeeded = mysql_set_charset( $charset, $dbh );
				}
				if ( $set_charset_succeeded ) {
					$query = $this->prepare( 'SET NAMES %s', $charset );
					if ( ! empty( $collate ) )
						$query .= $this->prepare( ' COLLATE %s', $collate );
					mysql_query( $query, $dbh );
				}
			}
		}
	}

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