Thanks, it works again! I had to edit 4 files (the 404's have a slightly different description).

I didn't archive posts yet when that topic was created. I scraped the posts later, the first post for instance is archived as oldposts #31577013. I have those posts only sorted by msgID, not by topicID. I'm currenly processing all data, but at the rate it's going, it's going to take a few weeks to complete. It's not very efficient, and therefore takes a lot of processing power.
I only now realize I should also process these posts per userID, and I should have done that from the start, so I'll have to restart this. It'll takes a bit longer again
Stuff like this would be so much easier from a database, but luckily I'll only have to do this once (more).