Hey there @Heraldo,
Just to keep all the relevant info in the same place, I've merged your post here to where you first commented.
As a first step, can you let me know if you've checked out Walter's reply here and let me know what you find?
Also, you mentioned that this occurred since you started using smart sync, can you let me know if your files in your Dropbox folder are local or online only? You can check this by looking at the sync icons next to your files/folders within the Dropbox folder on your computer.
Let me know what you find and we'll go from there - Thanks!