Strange to see the /webmail link to uebimiau is still there for new installs in httpd-alias.conf shouldn't that be changed to Roundcube for example? Now /webmail just comes up with a 404 on a fresh DirectAdmin install.
I've just updated custombuild to relink /webmail to squirrelmail, or to the webmail_link value from the directadmin.conf, if it exists.
So if you want to use roundcube, set it in the directadmin.conf.
The ./build rewrite_confs will respect the webmail_link value.
Please allow 24 hours for full sync of all files mirrors (it's only updated on files1 right now)