Use param raw as base64 can contain non alphanum chars

user-binding.php

@@ -11,7 +11,7 @@ use mod_labnbook\fetch;
 
 require_once \dirname(__DIR__, 2) . '/config.php';
 
-$returnUrl = \base64_decode(required_param('return', PARAM_ALPHANUM));
+$returnUrl = \base64_decode(required_param('return', PARAM_RAW));
 $courseid = required_param('courseid', PARAM_INT);
 
 $course = $DB->get_record('course', ['id' => $courseid], '*', MUST_EXIST);