I had the same problem. It happened to me as I was trying to run a character up to make Ambrosia. I tried three different characters and all three got to a point where they couldn't fish any longer. Then I made a connection. All three had just put the Death Flower in their inventory.
So I loaded one of my games and had the Sim plant the flower, removing it from the inventory. She could fish again. So, at least in my case, it seems to be having the Death Flower in your possession.
Cappy