Magento Connect - Invalid Post Data error

I have a partially completed website built in Magento. I can't take the site any further because when I try to add extensions using Magento Connect I recieve an Invalid Post Data error. I've tried to troubleshoot this in a number of ways (based on the suggestions by a number of people in the Magento forums - see link to forum posting below): * Checking for firewall blocks * Changing file permissions However I haven't had any luck! The Magento connect software was working when I first installed Magento and i'm not sure at what point it stopped working. I am using the latest version of Magento 4.1.1. I just need this error fixed so that I can continue to build the site. I can provide FTP/Admin/Cpanel access to the test site. [login to view URL];ie=utf-8&oe=utf-8&aq=t&rls=[login to view URL]:en-US:official&client=firefox-a#hl=en&safe=off&client=firefox-a&hs=40K&rls=[login to view URL]:en-US:official&q=+site:magentocommerce.com+magento+invalid+post+data&sa=X&ei=lqrjTJGlHonyvQP32Z3QDg&ved=0CAIQqAQwAg&fp=9ba797cf79e7a88d