Unable to start Apache after dist-upgrade on a Plesk server: undefined symbol: ap_proxy_check_connection

Symptoms

• After a dist-upgrade (from Debian 8 or Ubuntu 16), Apache fails to start with th the error message:

  # systemctl start apache2.service
  ...
  apachectl[14512]: apache2: Syntax error on line 146 of /etc/apache2/apache2.conf: Syntax error on line 2 of /etc/apache2/mods-enabled/proxy_http.load: Cannot load /usr/lib/apache2/modules/mod_proxy_http.so into server: /usr/lib/apache2/modules/mod_proxy_http.so: undefined symbol: ap_proxy_check_connection

• The same error message may appear in the Home page in Plesk.

Cause

The libapache2-mod-proxy-psa package exists on the server which is a leftover from Debian 8/Ubuntu 16:

# dpkg -l libapache2-mod-proxy-psa
...
ii libapache2-mod-proxy-psa 2.4.10-debian8.0.17100418 amd64 Apache Module mod_proxy

Resolution

1. Connect to the Plesk server via SSH.

2. Remove the package libapache2-mod-proxy-psa:

   # dpkg -r --force-depends libapache2-mod-proxy-psa

3. Disable proxy modules:

   # a2dismod proxy*

4. Start the Apache service:

   # systemctl restart apache2

5. Enable the default proxy modules.

   # a2enmod proxy proxy_fcgi proxy_http

6. Restart the Apache service:

   # systemctl restart apache2