Webmail is unavailable:
PLESK_INFO: Not Found
The requested URL /roundcube/index.php was not found on this server.
Apache Server at webmail.example.com Port 80
If a link webmail.example.com is opened manually in a browser, Plesk default page is shown.
webmail.example.com is resolved to a different IP address while it should be resolved to its domain IP address.
Make sure that webmail.example.com is resolved to the same IP that is specified at Plesk > Domains > example.com > Web Hosting Access.