"Giving WordPress its own directory" instructions not working in this case

Normally when I make a site for a client I do it all in theirsite.com/wordpress, and then when it’s time to go live I use the Giving WordPress Its Own Directory instructions to change the site url to theirsite.com.

I’m trying it again – unsuccessfully – with three factors I’ve never had in place before: 1) I’m using WP 3.2, and 2) the site’s url has a subdomain, wordpress.theirsite.com, and 3) the site uses a complicated purchased theme, Photocrati.

When I go into Settings –> General and change the site URL, I know you’re supposed to get errors back when you hit save, but I thought I’d note what they are specifically:

Warning: is_writable() [function.is-writable]: open_basedir restriction in effect. File(/) is not within the allowed path(s): (/var/www/domains/wordpress.theirsite.com/:/usr/share/phpmyadmin:/etc/phpmyadmin:/var/www/myfilemanager:/var/www/myinstaller:/usr/local/lib/php) in /var/www/domains/wordpress.theirsite.com/docs/wordpress/wp-admin/includes/misc.php on line 133

Warning: Cannot modify header information - headers already sent by (output started at /var/www/domains/wordpress.theirsite.com/docs/wordpress/wp-admin/includes/misc.php:133) in /var/www/domains/wordpress.theirsite.com/docs/wordpress/wp-includes/pluggable.php on line 934

When I finish the rest of the procedure, these two things happen: trying to load wordpress.theirsite.com gets me the site without access to the style sheet, apparently. Trying to go to the Dashboard at wordpress.theirsite.com/wordpress/wp-admin redirects me to the style sheet-less main site page.

I don’t know which of these factors – new WP version, Photocrati or subdomain – is causing things to mess up. Photocrati’s loosely-named “Support” section doesn’t address anything to do with this stuff – I can write to their support if it turns out that’s the problem. But in the meantime, can anyone take a stab at what the problem might be?