Hi,
it seems that your plugin UserMeta Pro(UMP) have some difficulties to handle multidomains configuration. Specialy when you use the multiDomains option with WPML plugin (http://wpml.org/2009/04/domain-per-language-subdomains-or-language-folders/)
Case studied :
On the login form, UMP load correctly the form, but when submiting, it send the data to the primary domain (ex: domain.com), and the one used on the translated version of the website (ex: domain.fr).
That’s why the “admin-ajax.php” script respond an empty response (security checking process probably)