When choosing a custom skin in the skin menu, the skin is invisible.  The game forces you to choose a skin from a skin pack, or one of the default skins.  If you try to join with a custom skin on, (which is invisible in the menu) the game doesn't let you join because the skin is invisible, thus invalid/corrupt.  For some odd reason, in the files of the game, it shows that your custom skin isn't invisible.  Please fix this issue. Look at the image below.