Diagnosing A network error occurred during your login request error

We have a user that is experiencing a "A network error occurred during your login request" error when they try to log into an email account via webmail for one account. I am not able to duplicate this from my system - the login works fine. The user is able to access the email account and webmail from another device, presumably on the same network - but I'm not certain of that. cpHulkd is not enabled on this server. The user's IP address is not blocked on the server via CSF or iptables. They are able to get to the login screen (further substantiating that the user's IP address is not blocked), but when submitting login details they get the "A network error occurred during your login request" error. I'm not finding anything in any of the logs on the server to indicate any issue. Nothing is being reported in /usr/local/cpanel/logs/error_log. /usr/local/cpanel/logs/access_log shows the login being POSTed but nothing else. How does one go about diagnosing where the issue might be?