Wordpress logs me out of the dashboard.

When trying to do certain things - including updating the wpmu dev dashboard, Wordpress logs me out, and takes me to the log in page.

I have disabled all plug-ins to see if one of them was causing the problem, but it doesn't seem to help.

I've tried this on IE, Safari, Chrome and Firefox and the account I'm using has administrator rights

Very frustrating as it isn't obvious what is causing it