If other sites are working fine then I suspect the problem is here:

The server has no place to write its session files.
If you have not already made the "cache" folder writable, please do so now so the installer can continue. Details on making the files writable can be found in the Installation Instructions or in the online FAQs.
Have another look at your setup, check permissions and settings. There are times, when one has 'root' access, that changes are made while not being the account Owner; but instead being 'root'.