I tried to access Online Accounts with the System Settings but it's just not there... I succeed to open it via terminal, but how can it be back with all the other System Settings icon?
It's definitely there, in system settings in the top "personal" section. If you still can't find it, open the Unity dash by pressing super and type "Online Accounts" without the quotes into the search.
There is a suggestion there: open the file
you'll need to be sudo in order to do it. Then find the line:
and change it to
Save&Exit. That's what did it for me, your online accounts icon should be back in the system settings panel.