I switched on error reporting and found this message - is this perhaps indicative of the issue?
Warning: Declaration of getuddeimTab::sendUserPMS($toid, $fromid, $subject, $message, $systemGenerated = false, $escaped = false) should be compatible with cbPMSHandler::sendUserPMS($toUserId, $fromUserId, $subject, $message, $systemGenerated = false, $fromName = NULL, $fromEmail = NULL) in /home/membersweb/public_html/components/com_comprofiler/plugin/user/plug_privatemessagingsystem(uddeimpms)/pms.uddeim.php on line 252