Installation Problem

Hi,

I've tried to install the punbb forum in a subdomain of my site www.tuttomanciano.com , the subdomain is forum.tuttomanciano.com the domain is registered with servage.

I've set the CMOD to 777 for the directory cache and the other requested, but when I can't set the Base Url of the forum because it send me this error

Error: Unable to write configuration cache file to cache directory. Please make sure PHP has write access to the directory 'cache'.

I've tried also all the possible url, how www.tuttomanciano.com/forum

The strange thing is that I can reach the forum from these two url www.tuttomanciano.com/forum and forum.tuttomanciano.com/index.php

Please hel me