A couple days ago I had a password issue, and I had to change it everywhere. The repeated login issue started right afterwards, so I attributed it to that.
Today I could not even compose a PM and submit it without the login popping back up. After logging in again it did NOT take me back to the PM Compose screen either. If happened so often that I finally used the browser "back" button to successfully return me to the Compose screen where I was able to Submit my PM.
I also cleared my cache, closed and reopened the browser which did not help. (Chrome on my Win7 PC)
Tonight at dinner I used my iPad and had the same issues with the rapid timeout.
Maybe they inadvertently changed the time-out in the cookies.