This is a known UI Bug.

There's a workaround mentioned in the thread above, but it's incredibly cumbersome. You can easily reproduce this error with no addons enabled by using /run OpenAllBags() then attempting to use an item in your bag.

The only fix for this bug right now is to revert to Bagnon version 10.0.10 until it's updated.

ye since last night i cant use the items anymore in my bags:

2x [ADDON_ACTION_FORBIDDEN] AddOn 'Bagnon' tried to call the protected function 'UNKNOWN()'.
[string "@!BugGrabber/BugGrabber.lua"]:480: in function <!BugGrabber/BugGrabber.lua:480>
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: in function UseContainerItem' [string "@FrameXML/ContainerFrame.lua"]:1409: in function ContainerFrameItemButton_OnClick'
[string "@FrameXML/ContainerFrame.lua"]:1461: in function <FrameXML/ContainerFrame.lua:1448>
[string "=[C]"]: ?

its not bagnon himself but bagnon item level

Getting this error even with Bagnon_ItemLevel Plugin disabled:

4x [ADDON_ACTION_FORBIDDEN] AddOn 'Bagnon' tried to call the protected function 'UNKNOWN()'.
[string "@!BugGrabber/BugGrabber.lua"]:480: in function <!BugGrabber/BugGrabber.lua:480>
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: in function UseContainerItem' [string "@FrameXML/ContainerFrame.lua"]:1409: in function <FrameXML/ContainerFrame.lua:1318> [string "=[C]"]: in function ContainerFrameItemButton_OnClick'
[string "@FrameXML/ContainerFrame.lua"]:1461: in function <FrameXML/ContainerFrame.lua:1448>
[string "=[C]"]: ?

Appears to be fixed in version 10.0.12 - thanks @Jaliborc. I'm looking into this specific taint issue more closely and can update you when I find something if you'd like. 👍

(although it would be great if Blizzard fixed this bug themselves)