When Baud Bag's bank is disabed (to get access to the Blizzard warband bank), and you then move an item inside the warband bank, the following error happens:

4x BaudBag/BaudBag.lua:55: attempt to index field '?' (a nil value)
[string "@BaudBag/BaudBag.lua"]:55: in function `?'
[string "@BaudBag/BaudBag.lua"]:265: in function <BaudBag/BaudBag.lua:261>
[string "=[C]"]: ?
[string "=[C]"]: in function `PickupContainerItem'
[string "@Blizzard_UIPanels_Game/Mainline/BankFrame.lua"]:970: in function <Blizzard_UIPanels_Game/Mainline/BankFrame.lua:969>

Addon version 11.0.2.1

I noticed another related issue:
If you use Warband Bank Distance Inhibitor while the bank of Baud Bag is enabled, it looks like you can move items in the normal bank, but you actually can't (it will generate errors). Blizzards UI only shows the warband bank in that case. I'd suggest that Baud Bag should at least show the normal bank bags as "offline" (like when opening them via the Baud Bag keybind) for now.
Later, if full warband bank support is implemented, it might even only show the warband bank like the Blizzard UI does.

Yea that's a shortcomming of the old addon structure and the fact that it doesn't support warband bank yet.
Will either fix it for next release (11.0.2.3) or have warband support ready, whatever comes first.

This specific error should be fixed with v11.0.2.3, but I'm pretty sure there will be more issues like this popping up. Will leave the issue open for the moment, if you find other similar issues please report here and I will see if I can put band-aids on them until propper support.

With 11.0.2.5-beta1 you'll find the first version with warband bank support. There are a few things still wonky but should already work pretty fine.

Minor: If you have the bank disabled in Baud Bag then the new warband bank frame will still be opened when you visit the bank.

Yea forgot to mention this in the changelog. The warband bank currently doesn't honor the deactivated state of neither warband bank, nor regular bank options.