Leeleepalomino The message that you can't download a world IS an error. But what I was asking is if you are separately seeing the "serious error" message covered in this thread:
[Current Issue] Sims 3 "serious error" when loading save | EA Forums - 8322938
I ask because I believe that's what the original poster's problem was, and you said you had the same problem, but now you're saying you don't get an error at all.
For the inability to install worlds, please see this guide, not the first post but "Best Practices" in the second post and then the entire third post for how to reinstall Store content into a clean folder and how to rebuild your user folder from there:
[Read First] Sims 3 Store guide | EA Forums - 11157679
Simply reinstalling your worlds into your existing folder won't help—you'll keep getting the same error.