None of the email accounts under a cPanel user (including the default account) can log into mail, even when attempting to log into webmail from the cPanel account. When attempting to use RoundCube, the user gets an authentication page.
This is due to LiveTransfer Proxy settings. You can confirm this is the issue by checking the dovecot log at /var/log/maillog for the following entry when attempting to access RoundCube webmail as an email account under the affected cPanel account:
nopassword set but password is non-empty
Run the following commands:
whmapi1 unset_all_service_proxy_backends username=cPanel_user