I will add my feedback for this error message from my recent experience moving a client to a new GoDaddy shared server. It was a problem updating to the latest 3.0.3 MODX version. And since it is a shared server, all I had to work with was my cPanel access and file manager.
I found that for me, the “Make sure your PHP session configuration is valid and working.” error was due to two things. First, the ‘session.auto_start’ directive was set to Off for my local value. Second, even with that set to On I could not get things to work. I had to update the sessions save path.
What I did with only file manager access was to edit the .user.ini file in the website root folder (public_html). I created a folder in my public_html folder and set my session.save_path directive to that folder in the web root’s .user.ini file:
session.save_path = /home/<SITE NAME>/public_html/<FOLDER NAME>
Lastly, in the same .user.ini file, I added the directive:
session.auto_start = On
as the first line.
Once I did that, I no longer received the error message when trying to install using the yoursite.com/setup path. Hope this helps someone struggling.