I see your rewrite rule starts like this, add_rewrite_rule( ‘^wishlist/ – so i did a test using this url wich works ok, https://www.villmarksbua.no/onskeliste/?wishlist_id=3 and if i change the whislist slug from /onskeliste/ to /wishlist/ then this url works ok, https://www.villmarksbua.no/wishlist/3/.

So whatever slug i am using the url fails, but when using /wishlist/ url works just fine.

Brgds
Rune