Another point is that I've had no problems upgrading to the latest version on an existing site, just get this error creating a new site starting with the latest version as installed by Softaculous.

Another error screen shot of the login screen attached.

I've seen this before, you might need to change permissions on your server to 'write'.

See if everything is set to 755 in your folder tree.

I've changed the startup folder and session.php PERMS to 755 and 777 without effecting the error msg.

permissions error messages continue after changing the folder/file permissions to 755 or 777.

Warning: session_start() [function.session-start]: open(/tmp/sess_b726c0bb0c4a7a862bd0d9ae0854cd07, O_RDWR) failed: Permission denied (13) in /home/bartsy/public_html/concrete/startup/session.php on line 22

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/bartsy/public_html/concrete/startup/session.php:22) in /home/bartsy/public_html/concrete/startup/session.php on line 22

Warning: Cannot modify header information - headers already sent by (output started at /home/bartsy/public_html/concrete/startup/session.php:22) in /home/bartsy/public_html/concrete/libraries/view.php on line 841

Hi,

This doesnt seem to be a sotaculous issue.
Your /tmp/ folder must be full.
Files are not being written there causing the error.

This is a new account and the tmp folder is almost empty. But I may not have the right technical chops to understand the constraint you are referring to.