Within my RoundCube howto, i just mentioned web1 as an example. In your case it can be different!
On a default ISPConfig server, the document root for Apache2 should be /var/www. When you're creating websites with ISPConfig, the webspace for each site are created within /var/www, like this:
web1, web2, web3 and so on. Within /var/www you will see also directories named like www.mydomain.tld
subdomain.mydomain.tld and so on. Those directories have symbolic links with the directories with names like webX. For example if www.example.com
is your first website created with ISPConfig it has a symbolic link with web1. So if your RoundCube website webmail.mydomain.tld is your 3th website, the symbolic link is pointing to web3 NOT web1 as mentioned within my 10 steps howto.
The website itself is not within web3, but in its subdirectory /web, so: /var/www/web3/web/