How it comes, really don't know. on other hosts it works correctly with this settings.

The problem:
If I login, the cookie is not set (correctly, see below, sometimes it is set, but not always). After login and browsing the forum is there is no problem, because the SID in the url (It is displayed in the url, so no problems are there). But, if your want to go to ACP, there is a problem.
You got the reauth screen, still no problem. You fill this in, submit it, and he say you are reauthed. After releading this page, the problem begin.
The forum say

Information

You do not have admin permissions and therefore not allowed to access the administration control panel.

And the SID is different in the URL as in the SID cookie. After deleting the SID cookie, you can go to the acp, WITHOUT reauth.

This problem is with different cookie settings:

Cookiename is always phpbb3_profiella
Cookiepath is always /
Secure is off.

By cookiedomain is http://www.profielland.nl" target="_blank, the cookie is set, but with the wrong url:
!http://profielland.nl/profielland.PNG
Note to the extra . before the www.

By cookiedomain is .profielland.nl or profielland.nl the cookie isn't set.

The problem occours if by using with and without www. in the url.