Hail Henk Holland
Thank you for your answer.
All what you said had been done, before ever I got the error messages.
The username, that I put into wp_config.php, is the same username, that is used for my website (I mean regular website, not blog, on the same server).
Is that the right value? Same way, the password. Why did I get the error messages?
What must I do, to make it to be right?
Thank you!