Edit: Just go here ([Addon] Jamba - An assistant for multiboxers. 12th Update - 0.4d released.) and download fan update 2 provided by DgtlSorcrs, it includes a fix for both this and the group invite issue.

Heres a quick fix for this.

Open Interface\AddOns\Jamba\Modules\JambaQuest.lua

Change line 1488 from

Code:
QuestWatch_Update()
to

Code:
--QuestWatch_Update()
Save the file then copy and replace it on each client install. All you have done is commented out the line QuestLog_Update(). It appears that the function has been changed by blizz in 3.1 and the easiest way to deal with it was just to remove it for now. Your abandon quest will still work but will not throw an error anymore.

- Aldercy