problem with album – page not found

I created 3 galleries.

Then put the widget in a sidebar, to show an album that contains the 3 galleries. In the widget the album is correctly shown. When click on a picture, it opens a page like mydomain.com/bwg_gallery/name

But the page does not show the images and says “Error 404 page not found”.

I modified that widget, to show a gallery and not the album. In that way the gallery is opened correctly. Then I re-tested, changing the widget to show again the album, and the problem with it remains: error 404 when click on an image in the widget.