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As noted in the linked FAQ, SHIFT+BACKSPACE (more specifically, LEFTSHIFT+BACKSPACE)

I would simply go to the problem toon and press that combo, and observe what the toon tries to do.
Can also do what the FAQ says, and on the problem toon, set SHIFT+BACKSPACE to some empty bind slot, the game should unbind it from wherever it is set, and then remove it again from the item you just set.

You got it bob, that solved it.

i was checking if Backspace was bound, when i should have been checking for Shift+Backspace

thanks so much for your time, much appreciated !