customers can´t log in as new customers in Zoho desk clients portal

customers can´t log in as new customers in Zoho desk clients portal

Hi

we have a clients portal with Zoho desk, and when customers try to log in as new users, they received the confirmation email but when they are redirected from their email to the clients portal in order to write the password and finish the process, they wirte the password BUT THEN the "registry button" doesn´t work and they can´t not register

please any suggestion

best regards