(Hella necro, but in the event someone else comes across this discussion with the same question...)

This is due to the game not being able to find the sprite sheet it was using for the player, a rather crude workaround to this would be to use another mod which also changes the player's sprite sheet mid-game. To return to the original skin however, you'd need one of such mods to have an option to point the game back to the default sprite sheet (prior to being disabled), which as of now I am not aware of any such mods that do so... With the know-how, one could use the mod Prospero2000 to manually direct the game back to the proper sprite sheet (by editing the players entity file in-game), but that's another discussion entirely.

V6 has this, wherever/whenever it is...