server/apps/files_encryption/files/error.php

<?php
if (!isset($_)) { //also provide standalone error page
	require_once __DIR__ . '/../../../lib/base.php';

	$l = OC_L10N::get('files_encryption');

	$errorMsg = $l->t('Your private key is not valid! Maybe the encryption app was re-enabled during your session. Please try to log out and log back in to initialize the encryption app. If this doesn\'t help maybe your password was changed outside the ownCloud system (e.g. your corporate directory). You can update your private key password in your personal settings to recover access to your encrypted files.');

	if(isset($_GET['p']) && $_GET['p'] === '1') {
		header('HTTP/1.0 404 ' . $errorMsg);
	}

	// check if ajax request
	if(!empty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {
		\OCP\JSON::error(array('data' => array('message' => $errorMsg)));
	} else {
		header('HTTP/1.0 404 ' . $errorMsg);
		$tmpl = new OC_Template('files_encryption', 'invalid_private_key', 'guest');
		$tmpl->printPage();
	}

	exit;
}