I do not have xdebug configured on that system, nor do we use Zend.
Turns out the domain mapping module wasn’t even needed for this site, but disabled the mapping didn’t change a thing.
I presume that the 100 nested functions stems from the fact that we have over 45 sites in our piwik installation, making the plugin do 45 times
[...]
[Thu Feb 07 17:51:48 2013] [error] [client 192.168.97.1] PHP 34. wp_piwik->callPiwikAPI() /var/www/wp.domain.tld/wp-content/plugins/wp-piwik/wp-piwik.php:867, referer: https://subdomain.domain.tld/wp-admin/options-general.php?page=wp-piwik/wp-piwik.php
[Thu Feb 07 17:51:48 2013] [error] [client 192.168.97.1] PHP 35. wp_piwik->addPiwikSite() /var/www/wp.domain.tld/wp-content/plugins/wp-piwik/wp-piwik.php:968, referer: https://subdomain.domain.tld/wp-admin/options-general.php?page=wp-piwik/wp-piwik.php
[...]
..but that’s just a wild guess.
And before that we have:
[Thu Feb 07 17:51:48 2013] [error] [client 192.168.97.1] PHP 7. wp_piwik->__construct() /var/www/wp.domain.tld/wp-content/plugins/wp-piwik/wp-piwik.php:1474, referer: https://subdimain.domain.tld/wp-admin/options-general.php?page=wp-piwik/wp-piwik.php
[Thu Feb 07 17:51:48 2013] [error] [client 192.168.97.1] PHP 8. wp_piwik->applySettings() /var/www/wp.domain.tld/wp-content/plugins/wp-piwik/wp-piwik.php:162, referer: https://subdimain.domain.tld/wp-admin/options-general.php?page=wp-piwik/wp-piwik.php
I could probably send you the complete traceback though.