Bug found and solved
My Wordpress installation is in a subfolder (
www.domain.com/_cms/) for security reasons, but still the site is accessed through the domain itself (
www.domain.com). Wordpress allows this, and there are separate settings for the two URLs in Settings > General. One of them is for the Wordpress installation URL, and the other one is for the website URL.
JobRoller, it seems, uses the Wordpress installation URL for its filter form when filtering our jobs by job type (full-time, part-time, etc.) , resulting in a "Page not found" message. I tracked the code for the form in /themes/jobroller/includes/forms/filter/filter-form.php:line 434:
433 if (is_front_page() || is_search() || is_front_page()) :
434 return trailingsla****(get_bloginfo('wpurl'));
435 elseif (is_category()) :
get_bloginfo('wpurl') should become get_bloginfo('url') and everything works just fine.