Hi Krileon and many thx for ur reply,
everything works fine since I changed the "access level" parameter.
After reading your answer, i wanted to look a little more...ans i found a bug with
a single member (administrator + registered),
even if there are several other members with these same rights, it's the only one that has this bug ( " Error decoding JSON data: Control character error, possibly incorrectly encoded" ).
So, I activate joomla error reports (max) and I have only one Notice "Notice: Constant K2_JVERSION already defined in /home/geaufrcujy/www/plugins/system/k2/k2.php on line 23"
and when I try to open the registration of the member concerned, I have a table that appears in the report (attachment):