a (or the) chink (or crack) in one's armour

   чьё-л. cлaбoe, уязвимoe мecтo; axиллecoвa пятa

    For years she had told hint how beautiful he was and now he could not live without flattery. It was the only chink in his armour (W. S. Maugham). I think I've detected the crack in her shining armour (J. W. Target)