This is not a bug. You need to move some items out of your default wishlist. If you have more than items in your default list, you cannot add anymore using the + sign until you move some out. You can still add things manually, but the + sign won't work.

No worries. I had momentarily forgotten that it did that too but oddly enough I was getting the error tonight too and I check the count on my default list xD. If you don't want to clear out your default list, you can make another list default for the time being and the + will work again (as long as it is below 500 items).

Locking this up since it is not a bug!