[A84]

Thank Rafa-el, my question was not if i had to change to something, but if i had to change just one, or both.

For those who might find this thread, the answer is:

- you must change them both. the /config.php file has "" assigned to cookie prefix, but apparently it is overridden as a default value somewhere. If you change it in /core/includes/config.php you have to change it in /config.php too (as stated in the config.php comment)

- in my case however, truncating the sessions table only did not work at all. In the end just to be sure, i truncated the table and changed both files. Hope this helps somebody in the future