On my test server I just ran the latest available upgrade script for Matrix 4 (automatic_upgrades_4-18-4.zip) to upgrade to version 4.18.9 and got the following error:
PHP Warning session_start(): open([SYSTEM_ROOT]/cache/sess_ecr2mks6ae0ca5h9utrpvmao01, O_RDWR) failed: Permission denied (13)
Unknown: open(/home/websites/squiz_matrix/cache/sess_ecr2mks6ae0ca5h9utrpvmao01, O_RDWR) failed: Permission denied (13)
Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/home/websites/squiz_matrix/cache)
Is this related to the upgrade script or something else?