Page navigation generates Not Found error

Hello all

Every time I try to open a category on my site, it opens fine. However, if I try to navigate using the “Previous posts” link, a Not Found error appears. What is going on and how can I fix this?

My website, in portuguese, is this: http://www.ricardoprado.jor.br
To test it, go up to “Portifólio” and choose one of the categories on the drop-down menu. The ones with a lot of posts have the navigation links below.

This is really frustrating since the site was set up completely by someone else, and I have no knowledge of WordPress to deal with it. So maybe it’s a simple thing, maybe not.

Best regards