Remembered one more thing - I tried to bind thumb buttons (Mouse4 / Mouse5), but it did not work.
Apart from that, by the way, the whole keybinding system is very cool. Only I already find myself rebinding keys depending on which class I play. It would be extremely cool if the system worked like an MMO/PoE, just giving you 1-2 action bars with configurable bindings and then letting you freely drag items or abilities into the action slots. And then saving your slot assignments per class! I realise this is a pain to implement, but it would be damned next level cool.