400: Bad Request

I am getting an "The request failed with HTTP status 400: Bad Request " Error in Report Manager Webpage. http://servername/Reports
The Reportserver itself and reports works fine (http://servername/reportserver).
Reports render OK and i am still able to connect to reportserver by management studio.

The environment is a W2K3 Standard with SQL 2005 (Build 2153) - and the server is a domain controller.

Reporting Service Configuration does not mention any issues - I have reapplied default settings - no change.
There are no errors in the eventlog.

I need some help - please

HANNESI have already checked the following posts

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=284929&SiteID=1
(I did not understend the "RSWebApplication.config" file - as soon as i write in the a reportserver url the website returns an execption - so this config must be empty I guess)

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=479636&SiteID=1|||This post gave me the finale hint (http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=165851&SiteID=1) - but I do not habe a specific IP address bound to the website - but I had specific hostheaders and it seems the reportmanager needs at least one binding with no hosthader in my environment - therefor I added a fifth binding with no hosthander, no specific IP and default Port

Hannes