Thank you for your reply, David.
I looked at the visitor table, and there is nothing special about the password field. So I went ahead and encrypted the password field of the login page. I then tried to log in, and I was redirected to the accessDenied page. After removing the encryption from the password formfield, the login worked again. So there is something I need to do to the database password field to encrypte it.
This is, what was said in the tutorial: "Because the database value will be encrypted, the compareble formfield will also need to be encryted."
How do I encrypt the database value?