[Typo3] Modern_Guestbook fails in redirecting after posting via form

[nomennescio]

I´ve built a little site for my gospel-choir containing one page with
modern_guestbook (list) and another with modern_guestbook (form).
The starting-point is set to the sys-folder with the guestbook-entries.
The redirect-page for the form is set to the page with the
member-area-entrance (one level above the guestbook-list).
 
After a while of perfect working now the redirect fails after sending a
new post: The system isn´t looking for the right page (member-entrance)
but looking for "localhost\" (backshlash!) or calling url-root followed
by an backshlash (!) and gives an error-message "page not found".
The new posting is although stored and can be viewed by looking into the
list-page.
 
I tried to uninstall and rebuild all guestbook-pagecontent, deleted
cache_pages and cache_pagesection, restarted MySQL (that procedure
helped a few days before), but now the same error comes up again and
again.
 
Don´t know what to do now ... :-(
Anyone able to give me advice?
 
the page is located at:
gospel-blankenese.ch.vu
testuser/-password: gospel/gospel
(go then to "Mitgliederbereich" - "Kommentare")
 
Thx a lot!
Holger