Plesk shows the error: Failed opening required ‘modules/firewall/FwConfigurator.php’

Symptoms

After logging in to Plesk or switching between "Power user" and "Service Provider" views, the following error message appears in a web-browser:

ERROR: require_once(): Failed opening required 'modules/firewall/FwConfigurator.php' (include_path=<...>

Cause

Plesk Firewall installation is corrupted.

Resolution

Reinstall Plesk Firewall:

1. Connect to the Plesk server via SSH.

2. Remove Plesk Firewall:

   # plesk installer remove --components psa-firewall

3. Install Plesk Firewall back:

   # plesk installer add --components psa-firewall