I have identified the problem... but have no idée of the solution.
After having done all the 5 steps you mentioned without success, I thought of one file that I had to modified : wp-db.php.
Indeed, I had a problem of character encoding. I have therefore a specific file where I added this
mysql_query("set character_set_results = 'utf8';", $this->dbh);
mysql_query("set character_set_connection = 'utf8';", $this->dbh);
mysql_query("set character_set_client = 'utf8';", $this->dbh);
I will check if the file has changed since 2.7, and try to enforce my workaround another time.
[Interesting site for newbies, esmi, by the way]