PHP Fatal error: Call to undefined function yarpp_get_option()

YARP 4.0.6 on WordPress 3.3.2+dfsg-1~squeeze1 (Debian)

Posting a comment on an article returns a blank page. This may or may not be related to YARP. Anyway, while investigating I found this sort of entry in the acces logs for every comment attempt:

2001:470:1f12:bfd::2 - - [28/May/2013:14:21:23 +0200] "POST /index.php/2013/05/28/no-anne-in-fifteen-years-my-insurance-will-not-be-tailored-to-my-genetic-risks HTTP/1.1" 500 20 "http://serendipity.ruwenzori.net/index.php/2013/05/28/no-anne-in-fifteen-years-my-insurance-will-not-be-tailored-to-my-genetic-risks" "Mozilla/5.0 (Windows NT 5.1; rv:20.0) Gecko/20100101 Firefox/20.0"

Looking at the corresponding entry in the error log :

[Tue May 28 14:21:23 2013] [error] [client 2001:470:1f12:bfd::2] PHP Fatal error: Call to undefined function yarpp_get_option() in /usr/share/wordpress/wp-content/plugins/yarpp-experiments/yarpp-experiments.php on line 364, referer: http://serendipity.ruwenzori.net/index.php/2013/05/28/no-anne-in-fifteen-years-my-insurance-will-not-be-tailored-to-my-genetic-risks

The function where arpp-experiments.php’s line 364 lies :

function yarpp_pingback_maybe_block_pingback($url, $post, $leavename) {
   global $yarpp;
   if ( !yarpp_get_option('experiment_pingback') || !$yarpp->cache->is_yarpp_time() )
      return $url;

So I disabled YARP Experiment’s “Avoid pingbacks to own site” option.

Now the error is :

[Tue May 28 15:17:11 2013] [error] [client 2001:470:1f12:bfd::2] PHP Fatal error: Call to a member function get_option() on a non-object in /usr/share/wordpress/wp-content/plugins/yet-another-related-posts-plugin/yarpp.php on line 54, referer: http://serendipity.ruwenzori.net/index.php/2013/05/28/no-anne-in-fifteen-years-my-insurance-will-not-be-tailored-to-my-genetic-ris`ks

And the function where arpp.php’s line 54 is :

function yarpp_get_option($option = null) {
    global $yarpp;
    return $yarpp->get_option($option);
}

Now what ?