well now you understand my confusion with seeing the error message =)
here is a link to the main page http://www.tugbbs.com/c_ads/index.php
here is a link to the category root that doesnt work http://www.tugbbs.com/c_ads/index.php?cat=31
clicking on any of the subcategories fails with that error displayed above: http://www.tugbbs.com/c_ads/showcat.php?cat=43
this category here works fine...and uses the same template as the one that fails http://www.tugbbs.com/c_ads/index.php?cat=15
any of its subcategories also works fine.
(each main category has identical subcategories for geographical region)
I have uploaded a screenshot showing the template options for the category that does not work...as you can see there are only two there.