[Lodur]

From zo_keybindstrip.lua:

function ZO_KeybindStrip:AddKeybindButton(keybindButtonDescriptor)
    -- Asserting here usually means that a key is already bound (typically because someone forgot to remove a keybinding).
    assert(self.keybinds[keybindButtonDescriptor.keybind] == nil)

So look for an addon that adds a keybind that the Mail InBox already uses maybe.

Or an Addon that adds a key bind to the Inbox and does not remove it so this asserts the second time you open the Inbox?