If your site logins or other functionality that relies on sessions was broken by a recent Plesk upgrade it’s likely due to permissions errors on the default sessions save directory. You can check this by tailing the log for the domain
$tail mydomain.com/logs/error_log
To fix:
- Create a new directory outside your webroot, owned by the user for the domain
- In “PHP Settings” for the domain, change session.save_path to point to the directory you just created.