Problem saving site_url in new context setting

I have been trying to set up a sub domain following the help guide

http://rtfm.modx.com/display/revolution20/Creating+a+Subdomain+from+a+Folder+using+Virtual+Hosts

I am trying to set the site_url context setting for the sub domain context and it will not let me save it as the sub domain URL, every time I click save it says that it is saving and then simply stops and shows the entry window again.

I can save other URLs (such as the main site URL) into site_url but not the sub domain URL that I am using.

can anyone please help me to solve this problem

MODx Revolution 2.0.8-pl (traditional)
PHP version 5.3.6
Apache version 2.2.17
MySQL version 5.1.56-log
Operating system linux

these are all the details for my hosting I can get without contacting my hosting company

I do apologize but I am just starting from the basics.

Have you setup the subdomain through your hosting provider first? This is one piece missing from the documentation.

Hosting Control Panel: Setup subdomain -> then follow the documentation provided.

I had an easier experience since my subdomains folders are under main (dir) folder

Base MODX - /home/public_html/(www.domain.com)
SubDomain - /home/public_html/mobile/(mobile.domain.com) (files in tutorial copied here)