Good to know, except my problem is saving Sims to the library, the Deathfish also disappear then to. I also discovered chests don't work in that situation, but maybe there is another solution to it.
Most likely that is the same issue actually. Town Transitions use the Import/Export system to move your sims between worlds, so any error during that process will also occur when you Import/Export to the Library.
If you are indeed encountering that error, the solution is the same as I mentioned over in the other thread : Read my signature.