Classified - edit ad link broken


I've installed the Classified plugin on my website.
I can add new ads. When I click on the link to edit an ad, it can't find the page.

The generated link is something like this:

My permalink setup is: /%category%/%postname%/

Every once in a while, the link generated is
which is correct and loads the edit form.

Why does the code generate an incorrect link most of the time for the edit button?