I'd gotten a 50x error page when trying to submit this post, too - but it went through anyway? This post doesn't show up in my threads page, though.
I read back your earlier comments before I tried posting - I know you have other pressing matters on your plate. As you can see, I just created this user at artemis.camp, so that I can get those notifications ... maybe?
I'm able to see that post, do you see it in private browsing / when logged out? If so, you might be blocking domains on kbin.social and removing them would make it appear again. That's been fixed in latest which hopefully kbin.social users should get soon, but wasn't affecting other instances I had checked so you should be fine to do so on artemis
I don't want to give false information, but on fedia the majority of 500 pages seemed to stem from the image media cache causing issues (our instance admin detailed the issue here). People were able to mitigate it by disabling images everywhere they could, but one place they can't be disabled is the user avatars on the notification page. I could be wrong, but it's my belief it's those images causing the 500s, and that pushed me to add https://codeberg.org/Kbin/kbin-core/issues/1039 in hopes that if it was possible to disable them the 500s would stop. All this could be wrong though, maybe it's something else like deleted messages or anything else, or maybe there's a better way to solve issues with cached images