Hello,
This is not a Plesk issue, but php related. I even wonder if it isn't joomla/mambo related because we didn't notice this behavior on other scripts.
Joomla/mambo will refuse to see the session safe path if your php version is 4.4.x or 5.1.x (possible 5.2.x, we upgraded from 5.0.5 to 5.2.4).
There are 2 possible solutions to the problem:
1) set the session safe path in vhost.conf and have it point to a directory within the virtual hosts httpdocs domain. Don't forget to 777 that directory.
2) set safe mode off
If you choose to set of safe mode off you also need to set of safe mode off in php.ini. Plesk enteres a safe mode = on in every single virtual host so it doesn't matter if its off or on in the php.ini.
You need to do this because of the virtual hosts, php sometimes has trouble to get the correct _ENV and will pass the _ENV from domainX to domainY
hopes this helps a bit
Jan