Observed on translatewiki.net for all actions "new thread" and "reply": Exception from line 544 of /www/translatewiki.net/w/includes/content/ContentHandler.php: Format json is not supported for content model wikitext
This means that LiquidThreads is completely broken at the moment. Setting severity to critical because of that.
I've tested on the Wikimedia deployment 1.21wmf3 (303c14d), and that is not broken.
Because of bug 41494, this is almost impossible to analyse.