I found that using a query cat=n & w=n works as "Plus" not like "AND".
So it actually does not boil down the outcome to just all of cat "n" in week "n" but using a week or a month and a category. I switched off all plugins and the behaviour does not alter so it seems to be system core. My question to the community or the developers: Is this bug or feature ? If feature, can it be altered and how ?
Best regards and thx in advance. BTW I am really ****ed here, because I build a business blog for exact the reason to have a good category support....:-/