[Thu Aug 04 19:48:08 2011] [error] [client 184.108.40.206] SoftException in Application.cpp:221: File "/usr/share/roundcube/index.php" is not in document root of Vhost "/var/www/domain.es/web"
[Thu Aug 04 19:48:08 2011] [error] [client 220.127.116.11] Premature end of script headers: index.php
Which php mode do you use for that website? And have you added the path /usr/share/roundcube/ to the allowed open_basedir paths of that website?
And I have tried to create a subdomain for redirect to roundcube but doesn't work:
Redirect Type: R, L
Redirect: Path: http://www.clientdomain.com/webmail/
Thats wont work as you acn not reach roundcube at the URL http://www.clientdomain.com/webmail
. Please see my description above on how to configure roundcube for that.