In the 'recent post' block, the inclusion of simplepie is incorrect in ImpressCMS 1.3.5.

in file 'reader/blocks/feed_recent_posts.php' change line 20:

require_once(ICMS_ROOT_PATH . '/libraries/simplepie/simplepie.inc');

into this:

require_once(ICMS_ROOT_PATH . '/libraries/simplepie/autoloader.php');