Based on your other post also, sounds like you have some permissions issue going on. There is no such bug in 2.6.8 with terms. First check and make sure that /components and /components/com_dtregister are 755 permission and files inside are 644. One way or another, this will end up being something specific to your site.
My tabs and popup calendars also do not work in the events backend - they work in the configuration page though. I found these errors in Firebug when inside an Event - are they related to dt or maybe I have a conflicting plugin? An upgrade to 2.6.8d didn't help.
this.dlist.getElements is not a function
[Break on this error] this.titles = this.dlist.getElements('dt');
tabs.js
($(filter) || document).getElementsBySelector is not a function
[Break on this error] return false;});return(nocash)?element...}else{temp.push('[@',param[4],']');}}
mootools.js