Hmm, could it have something to do with adult content?

I don't think so. Or, I should say, it fails on both non-locked posts with adult content, and on locked posts without adult content.

Also, have you viewed the LJ jounal with the browser you exported the cookies form? There's a per-journal cookie that needs to be set (unless that's what you meant by per-session cookie)

Yes, I have, and yes it is.