PHP Fatal error: session_start(): Failed to initialize storage module in v1.0.3

I have a modx site that has started to develop some problems, random pages don’t load, remain blank.
Server error log shows:
[client xx.xx.xx.xx] PHP Fatal error: session_start(): Failed to initialize storage module: user (path: /var/lib/php/session) in /var/www/vhosts/domain.co.uk/httpdocs/manager/includes/config.inc.php on line 60, referer: http://www.domain.co.uk/
May be associated with recent Plesk upgrade.
Site is running 1.0.3 & PHP Version 4.3.9
File space looks fine & permissions are still OK.
Can anyone suggest were to start looking?
With thanks.

I would check your basic session functionality, separate from MODx. Plesk is sorta a pain to work with...

Try creating one page with something like:

<?php
session_start();
$_SESSION[’test’] = ’Hi’;
?>

And another with:
<?php
session_start();
print $_SESSION[’test’];
?>

You should see "Hi" on the 2nd page, which would verify whether sessions are working -- remember try this on normal PHP pages. Leave MODx out of it.