Not sure why Mapster is listed as the offender in this error message, could be a red herring.

2x [ADDON_ACTION_FORBIDDEN] AddOn 'Mapster' tried to call the protected function 'UseQuestLogSpecialItem()'.
[string "@!BugGrabber\BugGrabber.lua"]:519: in function <!BugGrabber\BugGrabber.lua:519>
[string "=[C]"]: in function `UseQuestLogSpecialItem'
[string "@Blizzard_ObjectiveTracker\Blizzard_ObjectiveTrackerShared.lua"]:95: in function <...ObjectiveTracker\Blizzard_ObjectiveTrackerShared.lua:88>

Context: I'm on the Field Trial quest and tried to use the quest item on a (dead) mob.