I'm also having this problem ever since the forum updates, also with The Next Circle of Hell, Vol. 2 thread.
I'm using a Mac, and the issue happens on both Chrome and Firefox. I tried clearing my cache, etc. This happens on both my home and work laptop.
I think it's related to the fact that AW now logs me out automatically after about an hour, whereas it always used to keep me logged in. If I try navigating to The Next Circle of Hell via my bookmarked URL
https://absolutewrite.com/forums/showthread.php?306747-The-Next-Circle-of-Hell-Vol-2 while logged me out, I'll get the 500 error. Then I have to log back in and navigate to the thread from Forums > Rejection and Dejection > The Next Circle of Hell.
Sometimes if I try using the URL pasted above even while logged in, I'll still get the error (this isn't 100% replicatable, seems finicky). Being logged out, navigating to that thread, and getting the 500 error is 100% replicatable on my end.