The error comes up when trying to use file based session storage instead of database. Apparently, the configuration for what directory to store the session is not in the configuration files but in the database table "configuration".

If you upgrade your php to version 5.2.1, the database session storage breaks. Therefore, I had to switch to file system based storage. But the catch was that I no longer could log into admin as the session could not start. The configuration.php files did not have this constant.

The way to fix this is to log into your database and update the session directory record.

"UPDATE configuration SET configuration_value='/your/new/direcotory/' WHERE configuration_title='session directory';"

System Specs:
php.5.2.1
zencart 1.3.5
apache 1.3.37
MySQL 5.0.45