Error when updating to 2.8.1 or

Dear Ingenious WP Forum Members,

I’m trying to upgrade from 2.8 to 2.8.1. But when I click upgrade, the process stops with below error. I even installed (from scratch with fresh DB) previous versions (2.5 and 2.7) and still getting the same error. After all I installed 2.8.1 and tried upgrading the plugins and getting the same error. With previous versions too I get the same error, when I try upgrading plugins.

The error I get is:

Downloading update from https://www.ads-software.com/wordpress-2.8.1.zip.
Unpacking the update.
Verifying the unpacked files
Installing the latest version

Warning: posix_getpwuid() has been disabled for security reasons in /home/ROOT/public_html/MYDOMAIN.com/wp-admin/includes/class-wp-filesystem-direct.php on line 103

Please help me if you know why this is happening.

(ROOT and MYDOMAIN are replacing their original names)

Thank you.
Manoj.