So here's what I have seen when trying to log on today.
All of the login sessions are being performed by www.popsci.com.
All of the PPX activity is performed by ppx.popsci.com
I am showed logged in under every www.popsci.com page on the site.
I am never showed logged in under any ppx.popsci.com page.
When attempting to access any ppx.popsci.com page, I am redirected to http://www.popsci.com/user/login. However since I am already already logged into www.popsci.com, I am receiving the "Access Denied" error. It has nothing to do with not having access, it's simply saying that "you can't log in again."
So my guess here is that the problem either exists with the lack of authentication on the ppx.popsci.com server, or possibly with the fact that when logging onto www.popsci.com, a ppx.popsci.com cookie is created with the wrong information.
Anyway, just hoping that this can help someone.

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I don't think these people have a clue judged on their lack of response to the posts.
I have had the same problem, quite annoying.
I've been having the same problem since they revamped the website several weeks ago.
Clear your cookies then make sure you sign out when your done, I had the same problem and that is how I fixed it.
from Redwood City, CA
Same problem for me, what is mentioned above doesn't seem to fix it. I have tried multiple computers in multiple locations (at work and at home).