We just had this same issue. I tried several of the suggestions from this chain. My client’s site was hosted on GoDaddy and password protected pages would not work unless you let the page literally sit there for 30 seconds before putting the password in, then you might have to enter it a second time for it to work and let you through to the content. The developers of the original site found the problem had to do with the google analytics code that was on the page. The code was moved to the root of the site and this solved the problem. Please check your pages for google code and remove it before trying something drastic.
