“Previous entries” link changes and does not work

I created a blog at http://investigacion.udgvirtual.udg.mx/blogs/calacas and it was working fine til recently that the number of entries needed several pages to be displayed. It happens that the Previous entries link at the bottom of the main page sometimes points to http://investigacion.udgvirtual.udg.mx/blogs/calacas/page/2/ and works perfectly, but often changes to http://investigacion.udgvirtual.udg.mx/blogs/calacas/blogs/calacas/page/2/ and then does not work (Error 404 – Not Found).

I have discovered that if I log in then it works. If I log out, keeps working as far as I do not close the browser window. If I do the latter, it stops working because the link changes to the second one.

Any ideas?