Huh. That's incredibly weird -- none of the changes that we've made affected how threading is calculated, and the "parent comment" link is generated absolutely, not relatively (ie, it's not a case of your browser losing a referer header or referring information between windows). The only thing I can think of that might even remotely cause it are a few wacky edge cases... actually, I am pulling this theory out of my ass, but since you're an RP account, I'm gonna guess you use one of the cookie swap extensions like Login or Juggler, right? Try logging out of your account manually, delete the profile for that account in your cookie swap extension, create the profile in your cookie swap extension again, and then log in manually (not with the extension). Does that make it go away?