This has been happening since the Hack. AFAIK my ID and ISP were not harvested but I changed my password anyway.
I can access all boards and posts in the SN section except SNChat.
When I select SNChat on the PC, I get this link, absolutely every time. www.mumsnet.com/tempproblem.html
I use Firefox and also have Chrome installed. It happens on both browsers.
Today, I logged into MN on a Samsung tablet and got straight into SNChat.
Back to my PC, logged out, logged back in, clicked SNChat and absolutely no way could I get anything except www.mumsnet.com/tempproblem.html 
I emailed a friend who sent me a link to an individual thread on SNChat, that worked, but as soon as I tried to click on the Topics>>SNChat header, I get (bet you already guessed!) www.mumsnet.com/tempproblem.html
I'm stumped as to how I can access SNChat on the tablet but not the PC.
All help and advice very gratefully received 