Apache fails to start on Plesk server: undefined symbol: ap_unixd_setup_child

Symptoms

• The following error occurs on attempt to start Apache web server:

  PLESK_ERROR: httpd: Syntax error on line 353 of /etc/httpd/conf/httpd.conf: Syntax error on line 6 of /etc/httpd/conf.d/fcgid.conf: Cannot load modules/mod_fcgid.so into server: /etc/httpd/modules/mod_fcgid.so: undefined symbol: ap_unixd_setup_child

• After disabling mod_fcgid Apache fails with:

  PLESK_ERROR: AH00526: Syntax error on line 66 of /etc/httpd/conf/httpd.conf:
  Invalid command 'User', perhaps misspelled or defined by a module not included in the server configuration

Cause

The mod_unixd module is disabled on the server.

Resolution

It is required to enable mod_unixd in order to fix this issue.

1. Log into Plesk
2. Go to Tools & Settings > Apache Web Server
3. Select the mod_unixd module
4. Click OK
5. Go to Tools & Settings > Service Management and start the Apache Web Server service

Note: enable mod_fcgid back in case it was disabled during the troubleshooting.

Additional information

See the following article for the command line resolution: After dist-upgrade Apache was upgraded from 2.2 to 2.4, Apache and cannot start