/home/kloxo/httpd/webmail/roundcube/config/config.php is ONLY related to roundcube.
I'm talking about TLS to access webmail on Kloxo-MR.
So Kloxo-MR needs sto configure TLS certificates for webmail.mydomain.com and it should be mandatory as long as there is a TLS certificate for that domain.
I can take a look at this if you tell me which files are involved in setting up virtualhosts e.g
/opt/configs/hiawatha/conf/proxies/mydomain.com.conf
Check '/home/kloxo/httpd/webmail/roundcube/config/config.php'.
If you want involve for webmail feature, inform what's you do.