connection problem (member or admin)

Hello
I fully installed dolphin 7 RC, everything went well.
I would like to connect to the administration with the password and ID and I return every time the page identification.
If I want to connect to the site as a member, or even as admin, after stating the username and password, I fall on the page / member.php but it asks me to authenticate again. Yet if I go to the index.php part I appear well connected.
Is there to people who have the same problem as me? How to solve this problem?