When I want to add a trigger as a new condition for action, the popup window opens and I can browse through the list of my hosts and triggers. But when I click on the trigger - nothing happens. No error message on the screen nor logs. All the other action condition types can be selected without a problem.
The same functionality is working with Firefox 9.0.1, so I guess it must be some javascript error with IE8.

This bug appeared since Zabbix version 1.8.5, before that it was working with IE8 as well.