Try the solution given in this post https://support.plesk.com/hc/en-us/articles/213933205-Website-hosted-in-Plesk-is-displayed-incorrectly-shows-503-Internal-Server-Error-or-returns-a-PHP-warning-Failed-to-read-session-data-files
If you don't know how to do that ask your hosting provider to help you
Try the solution given in this post https://support.plesk.com/hc/en-us/articles/213933205-Website-hosted-in-Plesk-is-displayed-incorrectly-shows-503-Internal-Server-Error-or-returns-a-PHP-warning-Failed-to-read-session-data-files
If you don't know how to do that ask your hosting provider to help you
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