Bagnon has been blocked from an action only available to Blizzard UI. You can disable addon and reload the UI.
If you choose ignore the issue persists. Only work around I have found is to have the items you need to use out of your bag on you bar and it will work.

Which software were you running?
Cursforge 10.1.15

1.Open your bag
2. Try to use a consumable item
3. "Bagnon has been blocked from an action only available to Blizzard UI." Error will show.
4. select ignore
5. try it again issue persists.

I have tried disabling all other addons and still have the same issue.

Also note this is on Wrath of The Lich King Classic

I can confirm this is from less than 24 hours. Yesterday I had no issues.

Getting the exact same issue in Classic (not WotLK) today.

This issue can be created in 10.1.15 by my hunter with pet summoned:-

1. Open a dialogue with an npc trader, this simo opens your bags.
2. Attempt to feed pet with an item from your bags - error message appears as shown in original report.

There is such a problem on the retail version. It appears when opening the Boss Guide or when using the action bar with an open bag.

Just had it happen on my game too. Can I post something to help out?

Same thing happens on retail. Only seen it show up there after entering dungeons.

I have same issue on retail. It happens at random times. When this happens I'm unable to use any keybinds. If you click to ignore and hit any keybind for any action, the same error pops up. This persists until reloading the UI. I have grabbed a few of the errors from buggrabber:

1x [ADDON_ACTION_BLOCKED] AddOn 'Bagnon' tried to call the protected function 'MultiBarBottomLeftButton1:SetShown()'.
[string "@!BugGrabber/BugGrabber.lua"]:481: in function <!BugGrabber/BugGrabber.lua:481>
[string "=[C]"]: in function SetShown' [string "@FrameXML/ActionBar.lua"]:196: in function UpdateShownButtons'
[string "@FrameXML/ActionButton.lua"]:507: in function UpdateAction' [string "@FrameXML/ActionButton.lua"]:980: in function OnEvent'
[string "@FrameXML/ActionButton.lua"]:207: in function <FrameXML/ActionButton.lua:204>

Locals:
_ = Frame {
RegisterEvent = defined @!BugGrabber/BugGrabber.lua:487
0 =
UnregisterEvent = defined @!BugGrabber/BugGrabber.lua:487
SetScript = defined @!BugGrabber/BugGrabber.lua:487
}
event = "ADDON_ACTION_BLOCKED"
events =

1x [ADDON_ACTION_BLOCKED] AddOn 'Bagnon' tried to call the protected function 'PrimaryProfession2SpellButtonBottom:Show()'.
[string "@!BugGrabber/BugGrabber.lua"]:481: in function <!BugGrabber/BugGrabber.lua:481>
[string "=[C]"]: in function Show' [string "@FrameXML/SpellBookFrame.lua"]:1557: in function FormatProfession'
[string "@FrameXML/SpellBookFrame.lua"]:1600: in function `SpellBook_UpdateProfTab'
[string "@FrameXML/SpellBookFrame.lua"]:163: in function <FrameXML/SpellBookFrame.lua:142>

Locals:
_ = Frame {
RegisterEvent = defined @!BugGrabber/BugGrabber.lua:487
0 =
UnregisterEvent = defined @!BugGrabber/BugGrabber.lua:487
SetScript = defined @!BugGrabber/BugGrabber.lua:487
}
event = "ADDON_ACTION_BLOCKED"
events =

1x [ADDON_ACTION_FORBIDDEN] AddOn 'Bagnon' tried to call the protected function 'UNKNOWN()'.
[string "@!BugGrabber/BugGrabber.lua"]:481: in function <!BugGrabber/BugGrabber.lua:481>
[string "=[C]"]: ?
[string "=[C]"]: in function UseAction' [string "@FrameXML/SecureTemplates.lua"]:364: in function handler'
[string "@FrameXML/SecureTemplates.lua"]:690: in function <FrameXML/SecureTemplates.lua:672>
[string "@FrameXML/SecureTemplates.lua"]:704: in function <FrameXML/SecureTemplates.lua:697>
[string "@FrameXML/SecureTemplates.lua"]:746: in function SecureActionButton_OnClick' [string "@FrameXML/ActionButton.lua"]:100: in function TryUseActionButton'
[string "@FrameXML/MultiActionBars.lua"]:31: in function `MultiActionButtonUp'
[string "MULTIACTIONBAR1BUTTON10"]:4: in function <[string "MULTIACTIONBAR1BUTTON10"]:1>

Locals:
_ = Frame {
RegisterEvent = defined @!BugGrabber/BugGrabber.lua:487
0 =
UnregisterEvent = defined @!BugGrabber/BugGrabber.lua:487
SetScript = defined @!BugGrabber/BugGrabber.lua:487
}
event = "ADDON_ACTION_FORBIDDEN"
events =

1x [ADDON_ACTION_FORBIDDEN] AddOn 'Bagnon' tried to call the protected function 'OnOpen()'.
[string "@!BugGrabber/BugGrabber.lua"]:481: in function <!BugGrabber/BugGrabber.lua:481>
[string "=[C]"]: in function OnOpen' [string "@Blizzard_EncounterJournal/Blizzard_EncounterJournal.lua"]:586: in function <...zzard_EncounterJournal/Blizzard_EncounterJournal.lua:575> [string "=[C]"]: in function Show'
[string "@FrameXML/UIParent.lua"]:2938: in function SetUIPanel' [string "@FrameXML/UIParent.lua"]:2781: in function ShowUIPanel'
[string "@FrameXML/UIParent.lua"]:2646: in function <FrameXML/UIParent.lua:2642>
[string "=[C]"]: in function SetAttribute' [string "@FrameXML/UIParent.lua"]:3248: in function ShowUIPanel'
[string "@FrameXML/UIParent.lua"]:3209: in function ToggleFrame' [string "@FrameXML/UIParent.lua"]:1143: in function ToggleEncounterJournal'
[string "TOGGLEENCOUNTERJOURNAL"]:1: in function <[string "TOGGLEENCOUNTERJOURNAL"]:1>

Locals:
_ = Frame {
RegisterEvent = defined @!BugGrabber/BugGrabber.lua:487
0 =
UnregisterEvent = defined @!BugGrabber/BugGrabber.lua:487
SetScript = defined @!BugGrabber/BugGrabber.lua:487
}
event = "ADDON_ACTION_FORBIDDEN"
events =

It happens to me after a seemingly random period of gameplay when I use hotkeys for action bar selections. Same error, but I can click disable and it basically reloads the UI. I'm on retail. Started happening after the 10.2 update from Bliz.

Experiencing this issue, Season of Discovery when trying to feed my pet. Currently on the latest release of Bagnon

same issue SOD classic 10.2.9