Every time I try to claim the Rottgeist skin in from the Prize Shop, I get a popup that just says '500 Server Error'. I do have an available point to use (it doesn't get used up) and I have successfully redeemed the rest of the community prizes.

It shows as unlocked in the wardrobe now but the prize screen is still showing that I have point left to spend and i haven't purchased it.

Ive got the same issue and no clue why, but it i letting me use it

This is a known issue now, thanks for reporting! You aren't in any trouble for having the skin, you just won't be able to claim it until this is fixed which is low on the bug fixes totem pole. When we use this feature again in the future, itwon't be able to happen again!