I'm unable to access my site's dashboard as an admin user, but can access from other user accounts with lower privileges (e.g. editor). If I change that other user's status to admin using phpMyAdmin then the same problem occurs for that user - changing their status back corrects the problem.
When I log in as an admin user I get a 500 error. I've tried disabling all plug-ins, reinstalling WP 3.0.1 manually, and everything else that appeared relevant on the Codex login trouble page.
The server (webhost = Pickaweb) is running Linux kernel 2.6.18, WP 3.0.1, MySQL version 4.1.22, PHP 5.2.9.