Today my site was reachable.
Only the error „421 Misdirected Request.“ was displayed.

The problem and the hotfix is described here: https://support.plesk.com/hc/en-us/articles/33500191748887-Websites-hosted-in-Plesk-on-Ubuntu-are-not-accessible-after-a-recent-Apache-update-421-Misdirected-Request