OK, this is probably a rewrite issue. I am running Nginx on the server. This is what I have for the ads rewrites:
# ads rewrite
rewrite "^/ads(/|)$" /modules/cheetah/ads/classifieds.php?Browse=1 last;
rewrite "^/ads/my_page[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page last;
rewrite "^/ads/my_page/add[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page&mode=add last;
rewrite "^/ads/my_page/edit/([0-9]+)[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page&mode=add&EditPostID=$1 last;
rewrite "^/ads/my_page/edit/([0-9]+)/dimg/([0-9]+)[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page&mode=add&EditPostID=$1&dimg=$2 last;
rewrite "^/ads/my_page/manage[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page&mode=manage last;
rewrite "^/ads/my_page/pending[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page&mode=pending last;
rewrite "^/ads/my_page/expired[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page&mode=expired last;
rewrite "^/ads/my_page/disapproved[/]*$" /modules/cheetah/ads/classifieds.php?action=my_page&mode=disapproved last;
rewrite "^/ads/cat/([^/.]+)[/]*$" /modules/cheetah/ads/classifieds.php?catUri=$1 last;
rewrite "^/ads/all/cat/([0-9]+)/([0-9]+)/([^/.]+)[/]*$" /modules/cheetah/ads/classifieds.php?catUri=$3&page=$2&per_page=$1 last;
rewrite "^/ads/subcat/([^/.]+)[/]*$" /modules/cheetah/ads/classifieds.php?scatUri=$1 last;
rewrite "^/ads/all/subcat/([0-9]+)/([0-9]+)/([^/.]+)[/]*$" /modules/cheetah/ads/classifieds.php?scatUri=$3&page=$2&per_page=$1 last;
rewrite "^/ads/entry/([^/.]+)[/]*$" /modules/cheetah/ads/classifieds.php?entryUri=$1 last;
rewrite "^/ads/tag/([^/.]+)[/]*$" /modules/cheetah/ads/classifieds_tags.php?tag=$1 last;
rewrite "^/ads/calendar[/]*$" /modules/cheetah/ads/classifieds.php?action=show_calendar last;
rewrite "^/ads/categories[/]*$" /modules/cheetah/ads/classifieds.php?action=show_categories last;
rewrite "^/ads/tags[/]*$" /modules/cheetah/ads/classifieds.php?action=tags last;
rewrite "^/ads/all_ads[/]*$" /modules/cheetah/ads/classifieds.php?action=show_all_ads last;
rewrite "^/ads/top_ads[/]*$" /modules/cheetah/ads/classifieds.php?action=show_top_rated last;
rewrite "^/ads/popular_ads[/]*$" /modules/cheetah/ads/classifieds.php?action=show_popular last;
rewrite "^/ads/featured_ads[/]*$" /modules/cheetah/ads/classifieds.php?action=show_featured last;