I've got the same on Callisto 300. The solutions are:
1. To clean outside part of the button - in my case there was some glue-like material on it - and it prevents the button to move all way to inside.
2. To open back cover with 4 screws and to locate small button (as elecric component) on the PCB. It's possible to push it with small screwdriver to check either the problem is in outer (mechanical) button either with inner one (component). In my case it was outer one, and the problem was solved after cleaning.