I just upgraded a Wiki to 1.21.1 and installed LiquidThreads 2.0 on top of that (so I don't know if the problem occurs with earlier mediawiki-versions).
The threads seem to work fine. The only problem seems to be that
p-tags seem to be inserted between the
[ and the Edit/History etc button. No idea at which level this happens: MediaWiki/the plugin/the Skin.