The problem is not with MWG or the Web Server, but the web site. This always occurs when absolute links are used or the web site redirects to an absolute URL. The 301 you indicated could simply send Location: /nbtest/ but instead of that it sends the absolute URL to the browser, so the browser follows as requested.
On MWG you can catch a 301 response and rewrite it:
You read the header, write it into a variable, correct it and in the final rule delete the original Location header and inject a new one with a new value.
That's exactly what I need - Thanks!