Hi,
I am sorry but even with new user (registered yesterday) the params field gets changed when the profile is saved from frontend. And there is no very old rows in this DB as we began building this website about two month ago so with Joomla 3.6.5 and CB 2.1.2.
CB and Joomla are actually latest version and we have no third party extensions dealing with users except for CB and eventually Geommunity for CB but I think it only reads users nothing else (not sure).
I tried resetting one profile as you said and then edited it in frontend twice, this time with Chrome on Windows. Here is what I have in the DB : {"0":"{\"0\"
"{}\"}"}
I then edited the same profile in CB backend and here is the result : {"0":"{\"0\"
"{}\"}","language":"","editor":"","timezone":"Europe\/Zurich"}
Then back in frontend getting worse: {"0":"{\"0\"
"{\\\"0\\\"
\\"{}\\\"}\",\"language\"
"\",\"editor\"
"\",\"timezone\"
"Europe\\\/Zurich\"}","language":"","editor":"","timezone":"Europe\/Zurich"}
It seems each time I edit it, it adds something so I actually have no Json error but I presume it may change once the profile has been edited often enough and that's probably what happened with the user I encountered this error with.
Are you really sure this behaviour is normal ?