Login failed from client

Hello,

I have jazz installed on tomcat on a windows server. The (windows) name of the server is hostA, whereAs its name in the DNS is hostB. I can access the server from my computer using the DNS name only.
When I login from my computer (using url https://hostB:9443/jazz), I am redirected to URL https://hostA:9443/jazz/admin, which fails. If I then replace https://hostA:9443/jazz/admin by https://hostB:9443/jazz/admin, everything works fine, so I know the login was ok. A login directly on https://hostB:9443/jazz/admin works with no problem and no redirect to hostA.
One more problem is, that I can't login from the Jazz Eclipse Client, I get the error
Invalid user ID / password logging into 'tesat.it-informatik.de'.

CRJAZ0097I The request for URL "/jazz/service/com.ibm.team.repository.common.internal.IRepositoryRemoteService" was denied with a Forbidden status.

The tomcat logs do not contain any information.
I tried to solve the problem by updating tomcat's conf/server.xml to use hostB instead of localhost, but it did not help. (I updated

<Host name="localhost">

by

<Host name="hostB"> 

and the Engine's attribute defaultHost respectively).

Can someone tell me, where the problem is?
Regards,
Martin