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 can see the post when I go directly to it (with either the kbin.social or artemis.camp account), but it does not appear in my threads area under my kbin.social account (the one that posted it).
This a known bug, which has been fixed, but the fix hasn't been implemented on kbin.social.
The temporary work around is that you can't have and domains blocked. The link on e659668's post gives more details.
This will be fixed in the update planned for the end of September.