Thank you for all your help, JohnDoh.
I figured out, why it didnt work at the begin.
I'm using Debian etch 4.0 and a host panel (Confixx). This host panel has set a default
php_admin_value include_path .
for every host. Even though I created a new manually vhost (outside of the confixx vhosts) the apache configs were all set with this include path.
Of course it worked, when I changed my include_path with php_admin_value, but after the installation, Roundcube gets into problems with that.
So now I commented-out this entry in the causing configuration and now the stardard include_path from php.ini is active.
It works fine now! This thread can be moved or set as closed/solved.