Since two years ago, I have run into a strange problem. Earlier, I used the combination of option + s in order to type certain characters in my texts. It stopped working one day and I instead hear each time the kind of sound that you hear when you insert a wrong combination for a specific application. The problem is however independent of the application I am using (MS Word, TextEdit, etc) or the language selected. The problem is system-wide. I have tried the Keyboard viewer and the same happens there, i.e. no character output. The key on the keyboard at the level of hardware seems to function properly(you can type the letter s). I have checked the system preferences/keyboard for any eventual conflicts but nothing found. Is there anyone who has a clue about what the reason might be. Thanks in advance for your suggestions.