THANKS A LOT FOR YOUR EFFECTIVENESS IN REPLYING! The problem is now solved.
BUT I have a second problem with the "System's trash folder".
When I want to open it, it says "not possible because in phase of emptying".
But this is going on since yesterday!!!! (24 hrs).
Can you help ?
Have you restarted your machine since? If not, give that a try.
If you do not
need anything in your trash bin you can force empty trash from the terminal.
First open terminal which is located in Applications > Utilities > Terminal
Now copy and paste the following line into terminal exactly as written
This command is simply telling your computer to delete all
files and folders in your trash folder.
Log out and back into OS X and you should be good to go.