Website on Plesk server suddenly started to show 500 error: AH10292: Invalid proxy UDS filename

Symptoms

• Websites show the following error:

  AH10292: Invalid proxy UDS filename (proxy:unix:///var/www/vhosts/system/example.com/php-fpm.sock|fcgi://127.0.0.1:9000/var/www/vhosts/example.com/httpdocs/public/index.php)

  ---

  external Server Error

• Apache has been updated to a new version recently:

  Ubuntu 20.04:

  # grep 'status installed' /var/log/dpkg.log | grep apache2:amd64
  2021-09-27 12:46:57 status installed apache2:amd64 2.4.41-4ubuntu3.5

  Ubuntu 18.04:

  # grep 'status installed' /var/log/dpkg.log | grep apache2:amd64
  2021-09-28 06:25:55 status installed apache2:amd64 2.4.29-1ubuntu4.17

Cause

This is an Apache bug which arose with Ubuntu packages update and later fixed in the following versions:

• Apache for Ubuntu 20 version 2.4.41-4ubuntu3.6.
• Apache for Ubuntu 18 version 2.4.29-1ubuntu4.18.

Plesk uses Apache packages from OS repositories and does not control their version updates.

Resolution

Update Apache packages to fix the issue:

1. Log in to Plesk.

2. Go to Tools & Settings > Server Management > System Updates and click Recheck now.

3. Once updates are detected, click Update all.

The issue should be fixed, verify that websites are back online.