At line 1220 of multi-domains.php file, may I suggest replacing :
if ( $blog_id !== $userblog_id && 'login' == $action
if ( (int)$blog_id !== (int)$userblog_id && 'login' == $action
On http://fablab-lannion.org it was generating a css link on every page, even when located on the same site, which does not seem to be normal, as far as I can understand the way the plugin works. Moreover, the css link points to an URL that always gives a 404 error.