Session doesn't work after update from 2.2.1 to 3

I see that you have not specified the actual sess_save_path in $config.

I do not find any information about the default action of the 'files' driver if that path is NULL. There is a note that the final fall-back option attempts to use settings in your php.ini for session settings.

Try setting that absolute path and making sure that the directory has correct access permissions.