When customers attempt to sign up as affiliates, they may encounter the following error message:
Referral ID can contain only [a-zA-Z0-9_-] characters. something@something.com given
This error occurs due to the Username Referral IDs plugin, which sets the Referral ID based on the username—often configured as the user's email address. Since email addresses contain characters not permitted in Referral IDs, this leads to the error.
To resolve this issue, navigate to Configuration > Affiliate Signup > Fields and enable the option "Do not force email usernames" located near the username field.
Alternatively, you may simply deactivate the Username Referral IDs plugin to prevent this error.