<p>In our experience, it seems that this is a previously unresolved error on the part of Plesk that has to do with the PHP handler.</p>
<p><strong>Solution</strong>: Change the PHP handler of the respective web space back to PHP 7.2.</p>
<p><br></p>
<p>You can find more information about this error at Plesk e.g. More colorful:</p>
<p><a href="https://support.plesk.com/hc/en-us/articles/213413429-Website-running-PHP-FPM-shows-503-Service-Temporarily-Unavailable-failed-to-make-connection-to-backend">https://support.plesk.com/hc/e…ake-connection-to-backend</a></p>
<p><a href="https://support.plesk.com/hc/en-us/articles/213413429-Website-is-showing-503-Service-Temporarily-Unavailable-error">https://support.plesk.com/hc/e…orarily-Unavailable-error</a></p>
<p><a href="https://talk.plesk.com/threads/503-issue-on-website.354995/">https://talk.plesk.com/threads/503-issue-on-website.354995/</a></p>
<p><a href="https://support.plesk.com/hc/en-us/articles/115000320874-Site-does-not-work-on-PHP-FPM-on-Plesk-server-503-service-unavailable">https://support.plesk.com/hc/e…r-503-service-unavailable</a></p>
