This typically happens when the wishlist (not just the one page, but as a whole) has too many items to load in 30 seconds, so it times out.

There actually is an older report for this on the 🌟 Known Errors List - 11/9/14. If you don't mind, can you repost your report here: ☆ * Cannot Load Wishlists

I will make an update to the programmers that this is still happening. :)