To reproduce (since it could happen just occasionally).

• Go to ACP - General tab or ACP - Customise tab
• Turn OFF your internet connection
• re-check your phpBB version or Re-check all extension versions

 
The @ operator no more silences such errors since PHP 8

if ($socket = @fsockopen(($port == 443 ? 'ssl://' : '') . $host, $port, $this->error_number, $this->error_string, $timeout))