Thanks for the answers.
When I click on the lock,sometimes it "breaks",or,most of the times,it asks for my password,I give it,it opens,but there are no updates visible,although it has already told me there are e.g. "12 updates available".
From that point on,it does nothing and I close it down,since there is nothing to do.
P.S. I have a permanent Internet connection,so there is no chance the lock "breaks" because of no Internet connection.
The problem looks like Royo's problem,as described above.
MB:MSI 770 C-45 / HD1:500 GB-->Windows / HD2:320 GB-->Mint 17 Cinnamon
CPU:AMD X4 640 3.00 GHz / RAM:4.0 GB / VGA: NVidia GeForce GT630