When trying to signup as an affiliate, your customers can get this type of an error:
Referral ID can contain only [a-zA-Z0-9_-] characters. firstname.lastname@example.org given
This is caused by plugin "Username Referral IDs" which gets the value for Referral ID from the username, which is now set as the email address. The solution is to navigate to Configuration> Affiliate signup> Fiels and to turn on the option "do not force email usernames" near the username field.
Or you can simply deactivate this plugin...