See https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Collapsing_not_working
En.wiki is getting ".get() is not defined" type of errors in certain circumstances (so not always), suggesting that something that uses mw.cookie has not set its dependency correctly. As several parts have recently switched from jQuery.cookie to mediawiki.cookie, the culprit is like in one of these switch-overs. Please check.