In Upper Blackrock Spire an assertion failed at a line that Google doesn't seem to have indexed before, 4817:

Message: Interface\AddOns\Carbonite.Quests\NxQuest.lua:4817: assertion failed!
Time: Thu Aug 6 19:27:22 2020
Count: 1
Stack: Interface\AddOns\Carbonite.Quests\NxQuest.lua:4817: assertion failed!
[string "=[C]"]: ?
[string "=[C]"]: in function assert' [string "@Interface\AddOns\Carbonite.Quests\NxQuest.lua"]:4817: in function FindCur'
[string "@interface\AddOns\Carbonite.Quests\NxQuest.lua"]:9927: in function UserFunc' [string "@Interface\AddOns\Carbonite\NxUI.lua"]:6540: in function UserFunc'
[string "@interface\AddOns\Carbonite\NxUI.lua"]:3706: in function <Interface\AddOns\Carbonite\NxUI.lua:3688>

Removing the assertion and reloading UI gives instead a message something like "quest not in database". While I am engaged in Think of the Children, the click that provokes the error is in the normal list of UBRS dungeon objectives.

When investigating past revisions of this file, line 4817 was 4812 between 8.3.0.1 and 8.1.2, line 4811 between 8.1.1 and 8.1. and was 4810 at tag 8.0.1.4. For that line, 4810, I can find a matching error repost in issue #53.