Tok,
I've had the same problem off and on for the last several weeks trying to download things pertaining to SH 5 through Gamefront. It didn't matter whether I tried to go directly to Gamefront or tried to download through gamefront via Sub Sim's download section.
Up until the other day I had been using my longtime, reliable, Yahoo Internet Explorer. Several days ago I downloaded Mozilla Firefox which you can use without having to use it as your homepage thus avoiding having to change all your email addresses. I still use Yahoo for everything except downloading. Firefox downloads all our SH 5 MODs for me with no problem. In fact, I just finished downloading OH II v1.8.
You can get it here;
http://www.mozilla.org/en-US/firefox/new/
Good Luck