Valid rss not being displayed in the sidabar, getting warning

I’ve used

<?php
	include_once(ABSPATH . WPINC . '/rss.php');
	wp_rss('https://www.thefreepokerroom.com/forum/rss3.php', 5);
?>

to include a rss on my wordpress site, it has worked before as far as I’ve seen but now I get the error message:

<b>Warning</b>: array_slice() [<a href='function.array-slice'>function.array-slice</a>]: The first argument should be an array in <b>/home/........./wp-includes/rss.php</b> on line <b>890</b>

The feed I’m using seems to validate on feedvalidator.org and it’s displayed correctly in the browser so I’m pretty sure it’s got something to do with wordpress.

It still works occasionally but when it does updating the page seems to break it again.

I saw a french guy in a french wordpress community that seems to have the same problem, however I don’t understand it the solution.

I noticed the problems today when I added banners to the pages but even removing the banners made no change.. And they are just simple links anyway.

Does anyone know why this problem happens and how to solve it? Any ideas are welcome.