hmm, I must admit that I don't have any clue right now what causes this problem. Did you try if reading from this database works well? The error occurs at a place which actually does not have anything to do which is specific to RM - it directly occurs at the place where the connection is constructed and there is no specific magic done. So I would still assume that there could be an error at the settings. Here is the relevant parts:
<operator name="DatabaseExampleSetWriter (3)" class="DatabaseExampleSetWriter">
<parameter key="database_url" value="jdbc:mysql://localhost:80/travail_rapidminer"/>
<parameter key="overwrite_mode" value="overwrite"/>
<parameter key="table_name" value="table_elements"/>
<parameter key="username" value="root@localhost"/>
1. did reading from this database works with these settings?
2. port 80 is rather unusual for a database - the default for mySQL is 3306 if I am not mistaken
3. the user name root@localhost is also unusual - did you try "root" alone?
Sorry, I have no other ideas right now.