Upgrade to 1.5.10 shows blank screen for setup/index.php

The following information is required to get an answer:
Get this information from the PHP diagnostic (in the configuration wizard).
Gallery URL (optional but very useful): behind a firewall
Gallery version: 1.5.10
Apache version: 1.3.39 and 1.3.41
PHP version (don't just say PHP 4, please): 5.2.8
Graphics Toolkit: netpbm
Operating system: freebsd 6.1
Web browser/version (if applicable): opera 9.62/firefox

I've had to upgrade the php version for this machine and so now need a newer version of gallery1 which will run on php5. I installed gallery1.5.10 and went to the gallery/setup/index.php page but just get a blank browser window.

setup/phpinfo.php also shows a blank screen unless I comment out the require (dirname(__FILE__) . '/init.php'); and configLogin(basename(__FILE__)); and then it works. Doing a bit more commenting out in init.php shows it fails when it gets to require (GALLERY_BASE . '/session.php');

Apache error logs show nothing interesting, access log just shows and entry for me trying to load setup/index.php

All other php apps on the server continue to work.

Thanks for any help!