Oh dear – I dread seeing this error message.
It is almost always caused by an incorrectly formatted piece of data attached to one user’s details. But to find it you need to know your way quite well around using FireBug or whatever browser “developer tool” (usually F12 on your keyboard) you have.
If you can do that, then you can find the details of the json code that is failing. Then the error usually shows quite clearly.
Without that knowledge it will be pure guesswork.