In this case it seems to be a little bit different.
I've recorded with the help of Live Http Headers for both users host and portal admin. For the latter the cookie will be reinitialized.
For user "portal admin":
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http://www.domain.de/Providers/HtmlEditorProviders/TinyMCE/Includes/plugins/imagemanager/language/default.aspx?type=im&format=tinymce_3_x&group=tinymce&prefix=imagemanager_
GET /Providers/HtmlEditorProviders/TinyMCE/Includes/plugins/imagemanager/language/default.aspx?type=im&format=tinymce_3_x&group=tinymce&prefix=imagemanager_ HTTP/1.1
Host: www.domain.de
Referer: http://www.domain.de/testportal/Home/ctl/Edit/mid/460.aspx
Cookie: .ASPXANONYMOUS=LdVN8X6GyQEkAAAAYjY2MTJkZWMtYjY1Ny00MWRkLWI4M2ItOTM1NGYxOGVhMTc30; MCManager_im_lastPath={0}; hist=/domain.de/httpdocs/Portals/1/images/image.gif,/domain.de/httpdocs/Portals/1/images/domain_headline.jpg; language=de-DE; authentication=DNN; ASP.NET_SessionId=bgdfvde14tcq1j45yobfrr45; .DOTNETNUKE=EC5356...; portalaliasid=197A16B7C...; portalroles=7A571BF6...
Authorization: Basic bnNjaG5laWRlcjp6aFI0NUFxMlZrbjd0aQ==
HTTP/1.x 302 Found
Cache-Control: private
Location: http://www.domain.de/Providers/HtmlEditorProviders/TinyMCE/Includes/plugins/imagemanager/language/default.aspx?type=im&format=tinymce_3_x&group=tinymce&prefix=imagemanager_
Server: Microsoft-IIS/7.0
Set-Cookie: portalaliasid=; expires=Tue, 28-Nov-1978 10:44:14 GMT; path=/; HttpOnly
Set-Cookie: portalroles=; expires=Tue, 28-Nov-1978 10:44:14 GMT; path=/; HttpOnly
Set-Cookie: .DOTNETNUKE=; expires=Mon, 11-Oct-1999 22:00:00 GMT; path=/; HttpOnly
Set-Cookie: language=; path=/; HttpOnly
Set-Cookie: authentication=; path=/; HttpOnly
X-Powered-By: ASP.NET
Date: Fri, 28 Nov 2008 10:44:14 GMT
Content-Length: 308
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For user "host":
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http://www.domain.de/Providers/HtmlEditorProviders/TinyMCE/Includes/plugins/imagemanager/language/default.aspx?type=im&format=tinymce_3_x&group=tinymce&prefix=imagemanager_
GET /Providers/HtmlEditorProviders/TinyMCE/Includes/plugins/imagemanager/language/default.aspx?type=im&format=tinymce_3_x&group=tinymce&prefix=imagemanager_ HTTP/1.1
Host: www.domain.de
Referer: http://www.domain.de/testportal/Home/ctl/Edit/mid/460.aspx
Cookie: .ASPXANONYMOUS=LdVN8X6GyQEkAAAAYjY2MTJkZWMtYjY1Ny00MWRkLWI4M2ItOTM1NGYxOGVhMTc30; MCManager_im_lastPath={0}; hist=/domain.de/httpdocs/Portals/1/images/image.gif,/domain.de/httpdocs/Portals/1/images/domain_headline.jpg; language=de-DE; authentication=DNN; ASP.NET_SessionId=bgdfvde14tcq1j45yobfrr45; .DOTNETNUKE=31822EDA...; portalaliasid=49A8197...; portalroles=
Authorization: Basic bnNjaG5laWRlcjp6aFI0NUFxMlZrbjd0aQ==
HTTP/1.x 200 OK
Cache-Control: private
Content-Type: text/javascript; charset=utf-8
Server: Microsoft-IIS/7.0
Set-Cookie: portalaliasid=; expires=Tue, 28-Nov-1978 10:46:13 GMT; path=/; HttpOnly
Set-Cookie: portalroles=; expires=Tue, 28-Nov-1978 10:46:13 GMT; path=/; HttpOnly
Set-Cookie: language=de-DE; path=/; HttpOnly
X-AspNet-Version: 2.0.50727
X-Powered-By: ASP.NET
Date: Fri, 28 Nov 2008 10:46:12 GMT
Content-Length: 186
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User "portal admin" will be redirected (302)? In both cases it's the same Url.
Best regards
Niklas