@Lewlle EA has been working on the issue, according to the posts from an EA community manager in that thread. And the number of reports of this problem has dropped off significantly. So it's possible the issue was already fixed, but your existing content is preventing the game from handling any newly-installed content properly. That's speculation on my part, but it's easy for you to test.
Your purchased store content should all show up under Store > My Store Account > Purchased History. Your custom content can be moved to the new folder later. And if this test doesn't work, you can simply restore the old Sims 3 folder with no harm done.