Unable to login to Roundcube Webmail ( Hostname domain is the same as account domain )

Symptoms

When attempting to access Webmail, the page redirects to the login page. Errors similar to the following will be seen in /usr/local/cpanel/logs/login_log. 

CONFIG_TEXT: "POST /login/?login_only=1 HTTP/1.1" FAILED LOGIN webmaild: user password
hash is missing from system (user probably does not exist)

Errors similar to the following may also be seen in /usr/local/cpanel/logs/error_log.

CONFIG_TEXT: Cpanel::Exception::DomainDoesNotExist/(XID pje7hu) The domain “domain.com”
does not exist. at /usr/local/cpanel/Cpanel/AcctUtils/Lookup.pm line 63.

Cause

If the server's hostname uses the same domain as a domain hosted on a cPanel account, it will cause account domain lookup failures. The cPanel software doesn't support setting the same hostname as a domain hosted on an account.

Resolution

1. Log into WHM as the 'root' user.
2. Change the server's hostname via "Home / Networking Setup / Change Hostname"
3. Access the server's command line as the 'root' user via SSH or "Home / Server Configuration / Terminal" in WHM.

4. Run the following command.

   # /scripts/updateuserdomains

    

Additional resources

How to change the server's hostname from WHM
Effective Hostname Usage With cPanel WHM