Referral ID can contain only [a-zA-Z0-9_-] characters. email given

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. something@something.com 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...