Jul 17, 04:58 EDT
Identified - For customers experiencing a 421 error on their websites within Plesk on their servers, please note that Plesk is a third-party software. For assistance, we recommend visiting the official Plesk online help website, which should provide the necessary solutions and troubleshooting guidance.

Plesk have released an article with instructions on how to solve the issue:

https://support.plesk.com/hc/en-us/articles/33500191748887-Websites-hosted-in-Plesk-are-not-accessible-after-a-recent-Apache-update-421-Misdirected-Request