It's been like 3 days with this eerror which I thought was from fb's side. I had already a working comment box, but 3 days ago it started to show: Sorry, something went wrong.
We're working on getting this fixed as soon as we can. Going to the moderation tool page shows the same too http://developers.facebook.com/tools/comments . Curiously though, my comment boxes in other pages are working as supposed to. The affected page is just a personal one of mine: http://efaj.tumblr.com/
From what I've searched, I've only found either old information or unrelated
Yes it is a known new issue with Tumblr and Facebook,
I created a bug report about it here and if you want then you can subscribe to it.