The sick camera was a 550d. No second contact on the button (practically, You had to push it very hard).
After disassembling, I have found out that this is a new type of button (new for me), one side a little bit larger than the other. I decided to disassemble it, to see at first, what the problem is.
The contact plate is in fact formed of two similar plates – and the lower one, which is responsible for the contact has a darker colour – some oxide or what on the surface. They had NO BREAKAGE, NO sign of mechanical wear, so I reversed them. Fortunatelly the cap is a remountable one. After assembling the camera, everything was OK again.
As a conclusion, I advice You, that case the button is OK, but only is perfect, don’t desolder it and build in a new one, just clean it from oxides, as the flexible cable is not for many resolderings. In this cases the button can really be repaired.