Unable to start Apache after dist-upgrade on Plesk server: /usr/lib/apache2/modules/mod_proxy_http.so: undefined symbol

Symptoms

• There was a dist-upgrade from Debian 8 to Debian 9 that caused Apache2 to fail to start with the following error:

  # systemctl start apache2.service
  Apr 24 09:39:24 hostname systemd[1]: Starting The Apache HTTP Server…
  Apr 24 09:39:24 hostname 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
  Apr 24 09:39:24 hostname apachectl[14512]: Action ‘start’ failed.

apache2: Syntax error on line 146 of /etc/apache2/apache2.conf: Syntax error on line 2 of /etc/apache2/mods-enabled/proxy_fcgi.load: Cannot load /usr/lib/apache2/modules/mod_proxy_fcgi.so into server: /usr/lib/apache2/modules/mod_proxy_fcgi.so: undefined symbol: ap_proxy_check_connection 

• proxy module is enabled and proxy_fcgi module is disabled:

  # apachectl -M  | grep -E ‘proxy_module|proxy_fcgi’
  proxy_module (shared)

Cause

The libapache2-mod-proxy-psa package exists on the server which remained  from Debian 8:

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

Resolution

Note: if direct SSH access to the server is not possible, contact server administrator for further assistance.

1. Connect to the server via SSH;
2. Remove libapache2-mod-proxy-psa package:

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

3. Disable proxy module

   # a2dismod  proxy

4.  Start Apache:

   # systemctl start  apache2.service

5. Enable proxy and proxy_fcgi modules.

   # a2enmod proxy
   # a2enmod proxy_fcgi

6.  Restart Apache:

   # systemctl restart apache2

Additional information

Apache fails to start: ndefined symbol: ap_unixd_setup_child