I had set something wrong (or not deactivated) during the installation on the server. There was an automatic backup from Plesk. There was all my data in the subject line and you know how fast emails can be hacked and read.
What wasn't a problem of phpBB, but of my server settings or of Plesk, I don't know, because I have no influence on the server configuration.
According to the answer of the support this is so normal.
You have the MySQLDumper and luckily you don't need the automatic backup.