Migration to Plesk gone wrong

I’m migrating a couple of client’s blogs from a Dreamhost account that I’m closing to my VPS which is running Plesk Panel 10.4.4
(Not sure what version of WP – it was installed in April ’12. I don’t have WP admin access but canget it if necessary).

I copied all files and database to the new server. WordPress is installed in the docroot as previously. I’m using HOSTS to access the new install on my server.
– The database is definitely connecting (I initially had an error because I forgot to change MySQL server to localhost, fixed now).

I was getting errors:

[error] [client *.*.*.*] PHP Warning: require(/var/www/vhosts/*****/httpdocs/wp-includes/load.php): failed to open stream: Permission denied in /var/www/vhosts/****/httpdocs/wp-settings.php on line 21, referer: http://****/index.php
 [error] [client *.*.*.*] PHP Fatal error: require(): Failed opening required '/var/www/vhosts/****z/httpdocs/wp-includes/load.php' (include_path='.:') in /var/www/vhosts/****/httpdocs/wp-settings.php on line 21, referer: http://****/index.php'

This was only fixed by pushing permissions from 775 to 777 – unusual, because apache usually belongs to the group psacln on plesk. I’m not happy with this fix.
However, I’m now getting a blank screen with no source code – and no php error.
PHP memory limit is 64MB and there’s no delay in output.
The only other change I mage was that I didn’t re-upload about 50 themes the client had sitting there – I retained only twenty-eleven and the theme they currently have installed.

Any ideas?