Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Use Dropbox to Share Files
Dropbox is a freeware user-friendly app that enables you to share files quickly and efficiently. With this app you can access your files from another computer even without Internet connection– provided this computer also has Dropbox software installed. In addition, you can get access to your files even if there’s no Mac OS on another Computer, as Dropbox is compatible with Windows and Linux as well. Finally, Dropbox also offers you a free 2 GB of online storage space.
During the app installation you’ll be asked to create a Dropbox account if you don’t have one. You won’t be able to use the software without this account.
Once you install the app, open Dropbox and drop the files you’d like to share. If you have several computers with Dropbox installed, the files will be instantly available from any of these computers. You can use your Dropbox online account to access these files. And if you’d like to share files with your friends or colleagues, just click Share this folder so that they can easily view and modify these files using Dropbox. Finally, you can create online photo galleries using Dropbox that can be viewed even by those who don’t use this app.