As reported here: http://wordpress.org/support/topic/plugin-wp-less-issues-with-wp-network-site-and-domain-mapping

Hello, I´ve got an issue when enabling the bootstrap version for theme include, and WordPress Network Site(MU) with Domain Mapping(Free version).
It works fine before the Domain is changed(Less is compiled and css is stored), then i got error in line 96, in stylesheet.class.php, and i think the error is because of

$this->source_path = WP_CONTENT_DIR.preg_replace('#^'.WP_CONTENT_URL.'#U', '', $this->stylesheet->src);

• when the new domain is mapped.