[Zope-CMF] Unauthorized: portal_url trying to reply to a news item

[Robert Rottermann]

When I try to reply to a news Item I am asked for my credentials. (although
I am already logged in with Manager privileges)
When I cancel the dialog I get a traceback with
Unauthorized: portal_url

What could be the reason for that?
I am using CMFDecor.

Thanks
Robert

This is the whole traceback
Unauthorized

Sorry, a site error occurred.

Traceback (innermost last):
  File /var/lib/zope/lib/python/ZPublisher/Publish.py, line 223, in
publish_module
  File /var/lib/zope/lib/python/ZPublisher/Publish.py, line 187, in publish
  File /var/lib/zope/lib/python/ZPublisher/Publish.py, line 171, in publish
  File /var/lib/zope/lib/python/ZPublisher/mapply.py, line 160, in mapply
    (Object: discussion_reply)
  File /var/lib/zope/lib/python/ZPublisher/Publish.py, line 112, in
call_object
    (Object: discussion_reply)
  File /var/lib/zope/lib/python/Products/CMFCore/FSDTMLMethod.py, line 182,
in __call__
    (Object: discussion_reply)
  File /var/lib/zope/lib/python/DocumentTemplate/DT_String.py, line 546, in
__call__
    (Object: discussion_reply)
  File /var/lib/zope/lib/python/Products/CMFCore/FSDTMLMethod.py, line 177,
in __call__
    (Object: standard_html_header)
  File /var/lib/zope/lib/python/DocumentTemplate/DT_String.py, line 546, in
__call__
    (Object: standard_html_header)
  File /var/lib/zope/lib/python/OFS/DTMLMethod.py, line 188, in __call__
    (Object: standard_top_bar)
  File /var/lib/zope/lib/python/DocumentTemplate/DT_String.py, line 546, in
__call__
    (Object: standard_top_bar)
  File /var/lib/zope/lib/python/Shared/DC/Scripts/Bindings.py, line 338, in
__render_with_namespace__
    (Object: main_top.html)
  File /var/lib/zope/lib/python/Shared/DC/Scripts/Bindings.py, line 354, in
_bindAndExec
    (Object: main_top.html)
  File /var/lib/zope/lib/python/Products/PageTemplates/ZopePageTemplate.py,
line 278, in _exec
    (Object: main_top.html)
Unauthorized: portal_url