After restarting a server that has MDS on, I could no longer access MDS via the browser. This seemed very strange as I have restarted the server multiple times and it had never done this before.

Turns out the Application Pool had stopped for MDS. So I started it up and then it would stop again, frustrating me further.

Then is occurred to me that it was using my domain details and my password had changed. Then I went into the advanced settings of my MDS Application Pool to update my details under Identity.

And there we go, it works and I can access MDS again via the browser!

Hope this helps someone else with this error.