It only displays the language variable FSOCK_DISABLED instead of the text:

"'FSOCK_DISABLED' => 'The operation could not be completed because the <var>fsockopen</var> function has been disabled or the server being queried could not be found.',"

Should be fix with:
\phpBB3\phpbb\file_downloader.php:
Find:
throw new \RuntimeException('FSOCK_DISABLED');
Replace with:
throw new \phpbb\exception\runtime_exception('FSOCK_DISABLED');