Docman Extension

What do you mean a white page? Make sure you don't have a directory called "downloads" on your server.

I had a downloads directory...  :

Now there are some problems with url rewriting, i have a category1/gpu/
and a category2/gpu/ but the sef_ext rewrites both categories as downloads/gpu instead of downloads/category1/gpu and downloads/category2/gpu so when I try to acces  category1/gpu and category2/gpu i see an emtpy category because 2 different categories have the same url!

Yes well that's a limitation. You can not use 2 categories with same name, even if they are under different parent categories.

There's another thing, i cannot have a maincategory/secondcategory/subcategory

when i try to acces the subcategory i get a white page  :-[

Can I see example URL? There should be only one level of categories in URL.