When I try to choose a custom skin, I pick the skin file, and instead it shows an all black player model. If I try to press "Confirm" using the all black skin, I am instead sent back to the main menu without a custom skin.

To Recreate:

Press the button to open the skin select menu on the main menu screen. Select the Custom Player spot, and press "Choose Custom Skin". Once you select a skin, instead of the skin you selected, you will be met with a black player model.