If the MJ site doesn't work, and you can't get live chat to work, it sounds like a browser incompatability and/or you've disabled cookies, java script, etc. I would try with IE, and be sure it allows all the goodies like Java and cookies.
I've read occasional reports that not all ISPs have all of MJ's DNS entries in their DNS. (I had this problem at my employer too). When the MJ software can't resolve a hostname to an address, the software reports an error about being unable to connect to the internet. (But, I thought I was getting RC 3 or 4 when I had this problem.).
See if you can do an nslookup on the first three hostnames. And, on any of the proxies that are close to you.
If the hostnames resolved, I would bypass your router (if any). If you're running software firewall, I would turn it off.
If the 400 disappears, you can investigate how to enable them and let them play with MJ. I don't understand this topic because I have a router, and software firewall (overkill). I haven't had to do anything unusual for MJ to work. I haven't opened any ports. But, I've read others who said they had to do this. Or, they had to do it only for the initial install.