Lux unpickable

Just unlocked Lux, and i cant seem to play her. the client did let me pick her once just after i unlocked her, the other times a box pops up saying 'Error invalid_champ_selection' or she simply doesn't show in the pick menu. I've tryed to restart the client a couple of times, but it doesnt let me play her. Im playing team builder btw, dont know if the bug persists in other play modes