/usr/local/ispconfig/server/scripts/ispconfig_update.php

Update script hangs in most cases in ipv6-enabled systems, trying to retrieve version info file.

Server OS: Ubuntu 16 & 18

proposed fix

Use curl.

/usr/local/ispconfig/server/scripts/ispconfig_update.php

Replace line 94

-- $new_version = @file_get_contents('http://www.ispconfig.org/downloads/ispconfig3_version.txt') or die('Unable to retrieve version file.');

with

++ $ch = curl_init('http://www.ispconfig.org/downloads/ispconfig3_version.txt');

++ curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

++ new_version = curl_exec(ch) or die('Unable to retrieve version file.');