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.
Can I assume it’s associated with the new user account I just created?
Given that I was creating account just to test where password request is done (my brother gave me cause for concern), I will delete account. I have one pending new user to approve. If she submits requested information and I validate her, will see if that error occurs again.