Today SOE pushed a small patch to EverQuest II and with it came some UI changes one of which may crash your client if you have a custom eq2ui_mainhud_knowledge.xml
If you have the above file in your custom ui folder and you are experiencing crashes just remove (or rename) the file in your folder. Then keep an eye out for an update from the interface author.
I have posted a compare of this patch here
so you can see what code has changed in the default UI.